What system are you using, if you're using Pro Tools it will convert them if you bring them into the session, and make a copy of the file though retaining the original files at 44.1. What you could do is import all tracks into you recording program , batch convert them all and export them, this of course will take a couple of days probally, and you will need twice the amount of disk space.

you can pick up audacity, i believe it will convert files. I'd find an extra computer that isn't doing anything and use it for this purpose as I am guessing its going to take awhile.

alternativly you could stick with 44.1

I would stick with the 44.1 samples, and when you use them in your new songs, just convert the ones your using, as this is a pretty daunting task.

Upsampling to 48 isn't going to make anything sound better, and theoretically it could be worse due to rounding errors in the SRC math. As for your recorded tracks, I really think you should stick with 44.1 here too, especially if you're doing pop. Do a few tracks at 48. Do some at 88.2 and 96 if you have those rates. Listen carefully and compare. If you can't justify the additional data and system strain, don't use higher sampling rates.