Re encode files

I have a creative lab zen nano mp3 player (1 gig) which is suppose to hold 500 songs wma with average song being 4 minutes.
Over the past several months, I have recorded 432 songs into my pc. When I transfered them to my player, it would only hold 260 before I ran out of disk space. When I checked into this, I found that I should have encoded them at 64 kbps rather than 128. This would have allowed me to fit all the songs on the player.
So if anyone can help, I have two questions:
1. Wouldn't songs encoded wma 64 kbps sound like crap?
2. How can I re encode the songs from the present 128 to 64 without going back and copying all those tracks from my cds and making them the correct bitrate.

I use Roxio for doing this kind of stuff, but can't figure out a way to convert these existing files. Thanks

Thanks Cam, but I can't get it to convert files. A window pops up - Could not find the WmAudSDK.DLL FILE. Any idea on what this is?

So I would not be able to import into Itunes, re encode, and export out to a music folder and then transfer to the player. The songs I have don't have any drm in them now.
Also, from what I understand, with the wma files, you do not lose any quality when the files are compressed.