I didn't use the 4620 as now, some comming for me too. I do some test with those 2525. I got problem when using them with PICSTART. I'm not able to erase/program them correctly. This is a know issue by Microchip's team and they're working on a fix for the next MPLAB version... This is what they tell me last 2 weeks. Hopefully i have also a third party programmer :0)

New MPLAB in about 2 week... suppose to ;0)

After an FAST overview on both datasheet i don't see a real big problem to migrate from one to the other. Also, you have an internal oscillator that can be use with the 4620. I'll receive mine in a few.

Good luck, keep us posted on that!