There should be no problem. Just install Pleco, install the keyfile, and away you go. You may have to email Pleco (not sure) if you change your hot-sync user name. It's not a big deal. I imagine you could also get away with not doing so, but I'm not sure if that's a good idea or not. I don't imagine you'll actually notice any difference between the E2 and a 650/680. I'd recommend a 680, personally, as the 650's are getting pretty old, and you'll have a very hard time finding them new, if it's even still possible.
If you want to transfer your flashcards and user dictionary, you'll need FileZ to copy them from your internal memory to your SD Card on the E2, and then back again on to the 680's internal memory. I think they're called "Plecodict Flascards", "Plecodict User DB C-E" and "PlecoDict User DB E-C". I don't think you can transfer your preferences across, but it should be relatively easy to set everything up again.
Also, I'm fairly sure that the license only applies to one handheld, so you'll have to delete Pleco from the E2 once you've installed it on your new device.