http://usuaris.tinet.cat/mark/acnl_editor/help.html :-)
No, your old RAM dumps are not lost. They are not compatible with the editor, but the information is still there. You can truncate them to the new size at anytime by using an hex editor, and they'll become compatible.
It's still an experimental feature.
I've been bugged by a lot of PMs regarding problems with NAND values, lost savegames, etc. lately. And I don't want this to happen again because it's a pain in the ass. Sorry if I'm being too sincere, but I'm tired of whinings, insults and people wanting me to fix their faults.
So I just want to be sure that everything is safe. Acre editing is very risky, so I've added that warning button before starting. But still, I don't know the possibilities of editing outer acres at map/island. I remember reading a lot of people ruining their savegames (and a few blaming my RAM editor lol) while editing the acre map.