Can I transfer my 800k to a new bank account after using it for my Non-O visa?

Maybe you should use the Thai phrase for the “bank letter of guarantee” to be understood better

It is “rab roong thanakan”, and “for 2 months” is “fai song duean”.

The immigration is the “Tor Mor”. And as Stuart mentioned, and I am doing the same: I keep a colour copy of the "bank letter", so a year later the bank employee can refer to it, they even copy the complete text as was, and they see that the letter must be printed onto an original A4 sheet containing the bank letterhead in colour.

And by the way, you did not “renew my Non-O” . . . you do not have a Non-O visa any more since you used a 90-days Non-Imm-O visa to enter Thailand. The visa itself became invalid upon entry.

You applied for the renewal of your “1-year extended stay permit based on retirement”

I would be wary about transferring the funds personally as if anything goes wrong you are starting from the beginning, but that's not to say it can't be done.

If you wait 3 months and transfer 400k first to your new account wait until it shows in your new account then transfer the other 400k, I think that is the safest method.

I keep a copy of the information my bank issued to me, so that if they don't know what I require, I get the photocopy out and show them. This has worked so far (that could change, I realise).

BETTER ASK TOR MOR DIRECTLY FOR THIS