I was under the impression that you just needed to swap all the a/c lines to "barrier type" to convert to 134? Dunno if that includes the fittings. Also if it was retrofitted to 134 by a professional then I'm pretty sure they have to put a sticker somewhere stating so.
Either way, if you have r12 I'd keep it. If you enjoy your ac as much as I do it's worth it
'98 xj: 3.5" RE, 31's, nates front bumper, etc etc...
'90 xj: Wrecked, waiting for the day it can become a full blown trail rig.
'00 wj SOLD (still in family)