If you're on the extention of stay you might be able to change the reason for the extention without needing to leave the Country and apply for a non Imm B. I know it can be done the other way around non imm B to non imm O, ie extention for business/work to extention for retirement.
The vast majority of people retiring to Thailand arrive with the visa from their home Country which doesnt require 800k in a Thai bank thus enabling them to open a bank account without too many problems. They can then deposit and season the 800k for the 1 year extention of stay.
The people that post on here have problems, that's why they're posting, a very small minority of the expat/foreign community. I'd say the majority living here dont have any problems. I dont know anyone living here who had a problem opening a bank account.