Upgrading an external hard drive enclosure?

I have several terabytes-worth of external drives that I've acquired over the years. Most of them have a USB interface but as I'm sure many people know, transferring large amounts of data over USB repeatedly takes a LONG time and eventually it gets to the point where it's too much time. My most recent drive is connected over FireWire 800 and my next one will most likely be eSATA or Thunderbolt (if I can get a new computer). However I still have about 2 TBs locked up behind USB and though hard drives are much cheaper now, I would still like to find some way to upgrade the enclosure (or get a new enclosure) so I can keep using these drives.