If your embassy does not provide the income affidavit (many no longer will) your only option to switch to a “retirement” visa is using banked 800k.
For the initial visa and 1st year extension that’s the only option. For the 2nd and subsequent year extensions you can switch to the 65k international bank transfers method.
The money needs to have been in for 2 months prior to application. You need a letter from your bank (dated within 7 days) and an updated bank book same day of application.
Here’s a list for Phuket. I’ve included links for marriage or Thai child as you hadn’t specified which reason. Chiang Mai could well have slightly different requirements.
Yes it can be done if you meet the requirements. The main one being 400k in a Thai bank account. The list of requirements is quite extensive but varies from office to office so you’re best to go to the office and ask for the handout that lists their specific ones.