How can my aunt switch from an Immigration B visa to a retirement visa in Thailand without leaving the country?

Was she earning at least 65k a month for the last 12 months as proof of this could be used in place of the banked 800k?

Beside the Visa, tell her she should keep the Government Health Insurance, cheap as chips.

we normally don't allow these "I'm asking for a friend/relative" posts, because you aren't the person getting the new extension but this is the general way she'd do it

She should go to the immigration office soon and get the hand out listing the requirements for a yearly extension based on being over 50 (retirement) so that she knows what documents to get together

She gets the financial requirements together (800K baht in a thai bank account in her name only for 2 months before she applies)

She gets the termination paperwork from her employer, she cancels the work permit, gets the documentation together for the retirement extension, goes to the immigration office, cancels the extension she's on now (she's not on any Non-B VISA, she's been on yearly extensions) and as soon as the extension is cancelled she immediately applies for the new extension based on being over 50 (retirement) for 1900baht. It's granted right then.

She doesn't need to leave the country for changing the reason for an extension from employment to retirement at all.