After transferring to a newer HUD, the old HUD jumps onto me and becomes and "*OLD* HUD" and SL won't let me rez it again.

Yup, one of my favorite SL inventory bugs.

Just relog and it'll rez ok. Someone also reported that attempting to wear it first (then detaching it and then rezzing it on the ground) also worked, but your mileage may vary.

BTW, the *OLD* HUD will work fine (it's not gimped or anything), though it might nag you little a bit just so you know that you aren't wearing the newer HUD.