Yes and no.
The new editor will import your old terrain.
It will import all the old files.
You will end up with square coastlines and the ground types will look like a checker board. Both will need extensive modification but the system is designed to let you use a graphics program to do all the editing.
Your old textures will need extensive work as well because they were designed to cover one square mile and now they must cover four square miles. A bush or tree in the texture will be four times bigger then before!
Old expression: The devil's in the details.
PM me if you need more assistance.
Edit:
Oh, and in the AH1 terrains, the custom work was done in the TE. That's not so any more so if you had custom airfields etc, you'll need to use the OE now. I'm making a guess here that you're talking about your Israel(?) terrrain.
