As others have said, the only thing required
is that you notify the FCC of your new mailing address. And operation as either W7/KL2NW or KL2NW/W7 is a personal preference (unless required by an operating event like a contest), both in terms of whether it means portable/mobile or just that you're not in Alaska any more. You'll see stations operated with and without, portable/mobile and fixed. I'd personally recommend making your own decision and then sticking with it. Don't ask for a consensus opinion from the crowds because you won't get it.
As for where on the FCC site, it's a little tricky--they don't explicitly say it quite this way. Under (Amateur) Sequential Call Sign System
- "Each new sequentially assigned call sign is sequentially selected from the alphabetized regional-group list for the licensee's operator class and mailing address."
- "The station is reassigned its same call sign upon renewal or modification of its license..."
The second one is the "rule" that says that your callsign doesn't change when you change your address, even if your callsign was originally assigned to an Alaskan address (unless you accidentally check the "assign new sequential callsign" box!)