Had retirement visa for the last two years. Legally married in Thailand with one child.

Do not have 800,000 baht in bank this time. Letter from German consulate about income of 40.000baht will be enough instead of the 800,000baht? What is necessary to switch to marriage visa?

The wife, her ID card, her home register, birth certificate (child my get you points as officers do like a break from routine), photos together in home and outside with number visible (at some places), map to home (at some places), a passbook savings account and letter of account balance will probably be asked for. And of course that Embassy letter. Also be sure your 4x6cm photo for application is new.

The wife, her ID card, her home register, birth certificate (child my get you points as officers do like a break from routine), photos together in home and outside with number visible (at some places), map to home (at some places), a passbook savings account and letter of account balance will probably be asked for. And of course that Embassy letter. Also be sure your 4x6cm photo for application is new.

And a marriage certificate?

Correct - very important item. And a copy.