Now less than 90 days from retire visa renewal and would like to transfer the 800,000bt from regular savings to "fixed account" for better interest rate and ability to borrow against it. Is it ok to have that 800,000bt go from 1 account to another in the same bank within the 90 day period or does it have to stay in the same account?

That would all depend on the Immigration Officer you deal with.

Personally I would not push my luck.

The exchange rates are so small why not wait until your application is accepted before moving the cash.

Up to you though.

Just learned that "fixed account" can be borrowed against. Therefore those funds could be put into gold for example rather than sitting in cash. I don't know, but suspect, gold is going to fare much better than cash, considering current events.

I agree with LB, wouldn't push my luck for a few extra bob!!

Stay as you are, at least for 90 days.