Can one spouse switch from a retirement visa to a trailing spouse visa while the other remains on a retirement visa in Thailand?

Before you need new extensions both you and your husband need to slog on over to your immigration office, get the handout listing the requirements for "trailing spouse" extension and make sure you can meet them.

There are no financial requirements for the "trailing spouse" extension, you're 'piggy backing' on to a primary extension holder who does meet the financial requirements.

Lucky your immigration office allows joint accounts, usually they require double the financial requirements if they're going to allow a joint account and I don't know any couple who each have a retirement extension based on monthly income who use joint accounts here.

Again, you need to talk to your immigration office WAY before you're thinking of switching reasons for one of you from retirement to trailing spouse..

Take your marriage certificate too so they can tell you what has to happen for that to be accepted by the immigration office (most have to go thru some certification process which depending on country can be convoluted)