You can certainly do the retrofit now as you have replaced most of the stuff, that may be problematic with the change. The mineral oil does not have to be removed. It is compatible with the Polyol oil that is used with r134a. Pull a vacuum and charge away. I did the retro on my 89, and thus far this summer it has worked quite well. A bit slower on cool down when it's hot, but it's really not bad considering the system is not optomized for that refrigerant. BTW, do NOT use PAG oil for your retrofit. You will have to change all of the hoses and get the barrier type hose.
2000 Dodge Grand Caravan SE
[This message has been edited by jeffsr (edited 09-12-2000).]