What you don't say is whether your new system will be a desktop PC or a laptop. Assuming it's a desktop, then you can get a PCIe plug-in FireWire card for about $25, and that will allow you to continue to run your M-Audio 2626, at least in the short term while you put more consideration into updating your audio path. Make sure the FireWire card you choose has the Texas Instruments (TI) chipset. The FW card I got a while ago was the Lycom PE-101, which I believe is still available, but there are others.

Noting that the M-Audio 2626 does not alter or otherwise affect any audio sent in via its ADAT optical or S/PDIF RCA ports, you could widen your choice of new audio hardware devices to include digitising pre-amps that have S/PDIF (2-channel) or ADAT (8-channel) outputs, using the 2626 as the computer interface for these digital input signals. There would be easy expansion to 10, 16 or 18 channels at standard sampling rates by adding further 2- or 8-channel boxes, as well as having the option to fall back on the analogue inputs of the 2626 for secondary sources.

When it comes to retiring the M-Audio unit, a little care in the choice of a new audio interface would allow you to plug the ADAT 8-channel box into it to give 16 channels. There is an advantage in not having the same make or model of hardware for all your inputs to give you some sonic flexibility when assigning channels for audio sources and microphones.

Think about what course you might take, and we will be happy here to give constructive comments.

The only thing about the m-audio is that they are not very good on driver updates and support. Keeping it in your setup may add a futur risk factor.
Other than that, I think it's a great Idea to keep it and look for an Adat 8 channels preamp unit. But at 300-400$ isn't going to be quality priority gear.

ADAT Preamps :
https://www.sweetwater.com/store/detail/ScarOctoPre 400$
https://www.sweetwater.com/store/detail/ScarOctoPreDy 600$
https://www.sweetwater.com/store/detail/ASP800 800$ (this is the one I would get, way better preamps)

Interfaces :
The PreSonus 1818VSL isn't a bad choice. I think DonnyThompson has one and he gets good results with it.
To expand to 16, you would use the adat but you must run at 48khz max to get 8 adat channels at 96khz it's only 4 working channels.
https://www.sweetwater.com/store/detail/Scarlet18i20G2 500$ also have the same Adat limitations
Other choices :
https://www.sweetwater.com/store/detail/Studio192 900$ (it has 2 adat inputs so 8 channels at 96khz and 16 channels at 48/44khz)

The Presonus 1818 is a good i-o, it's got decent preamps (8) and good converters.
It is expandable through optical (adat lightpipe) so you could connect another pre amp to it that had the same opticals. There is also a word clock connection for when working with multiple digital devices.
The first two inputs are mic/instrument, the remaining 6 are mic/line. Each input has its own gain control. There is a headphone jack with volume for monitoring during tracking (or playback). There are two control room outs on the back, and an additional 6 outputs (1/4" balanced).
I've found the pre amps (X-Max) to be fairly transparent. Not as clean as preamps like Millennia, Grace or Apogee, which are all "uber clean", but still pretty clean. If you are looking for color/character in a preamp, this won't really get that for you.
I think it's a great bang for the buck, I've been able to get good results with it, but it's not the best pre-i /o available - though it is perhaps the best one in its price class for overall sound and features. It's capable of recording up to 96k/24 bit, though I usually stay at either 44/48.
FWIW
-d.

Addendum:
If you do go with the 1818, you'll want to check Presonus's site for driver and firmware updates.
When updating firmware, don't have any speakers or headphones connected. There are firmware updates that will occasionally have bursts of noise or tones to them. I'm not saying this is for sure, and you may not even require a firmware update, but if you do, don't have anything other than the USB cable connected while this process happens, just to be safe. You don't want to blow up your monitors or headphones if your volumes are up, and the best way to avoid this is to have them disconnected in case this happens.
-d.

The 18181VSL ( I haven't used one, but I have a Studio 192 and love it) looks like it offers a lot for the money. The company support and driver development is, imo, the first thing one should look at when deciding what company you are going to give your money to. I have had no issues with Presonus support.

FWIW, I've never had much luck with anything digital that Tascam makes.
Their digital gear has always sounded weak and thin to me. I don't know what converters they are using, but I'm betting they're not as good as what Presonus or Focusrite offers.

Their preamps are pretty weak, too, gain-wise; condensers work fine because they get power from the 48v supply, so they don't require as much gain as dynamic mics or ribbon mics... so, if you were using a low output mic - such as a Shure SM7, or pretty much any Ribbon mic, the Tascam probably isn't going to give you enough gain to run those types of mics optimally, and you'd likely have to add an inline preamp "gain booster", such as a Cloudlifter or Cathedral Pipes Durham, to get the signal up to optimal levels. Yeah, technically, you would get signal to your DAW with those mics without a booster, by cranking the gain on the Tascam, but Tascam preamps are also notoriously noisy, even more so when you push them.
Drivers were hit and miss with Tascam ( for me). I had an 8 channel model several years ago and it was... well ... it sounded pretty weak and thin, lacked gain, and gave me driver issues more than just a few times.

IMO, the Presonus 1818 is probably your best bet for preamp i-o's in that price class.
It expands via adat optical very easily, and provides a word clock for syncing digital devices. It also provides 8 1/4" TRS outputs ( other than the two master outs), should you desire to do re-amping, or send tracks or stems out to an external mixer, summing device, or other analog processing gear.

I have no experience with M-Audio preamps/i-o's, so I can't comment.
-d.

Mark Jeghers, post: 454656, member: 51059 wrote: I want this: 8 inputs XLR/line with an affordable path to expand to 16. Needs to be USB 2 or 3.

Just remember that if you go with the 1818VSL and want to record 16 tracks (8 from ADAT), you will need to record at 44/48khz.
96khz will give you only 4 ADAT inputs.

Of course this is not a problem for most people but thought it should be said...
Apparently again ;)