Can I Switch from a Multi-Entry Non-O Visa to a Single Entry Visa with Extension in Thailand?

I'm currently on a multi-entry Non-O (based on marriage), last entry March 2nd, valid until May 30th. I usually travel frequently, so this visa seemed sensible at the time I got it.

I'm now considering to switch from the multi-entry Non-O to a single entry + 12 months extension, since I'm worried that (a) the amnesty may end sooner than July 31st and (b) that even if it doesn't it may still not be easy to re-enter Thailand after leaving unless one holds a work permit.

Questions:

1. Is it possible to do this switch in country at all? I'm planning to do it ASAP as I think doing it while already on the amnesty will be too much of a headache/outright impossible.

2. Regarding the 400k in Thai bank account: until last December I held work permits for 6 years running, so I have more than the necessary amount in my account (and have had it for longer than the required 2 months). Can I just use my existing account? Or are there some special rules, like the money needs to be transferred in from overseas or anything like that?

Thanks!