Can I use the 400k baht in my bank after getting a NON-O visa based on marriage and still qualify for a retirement visa later?

Why change? A type O extension based on Thai wife is much better than a type O based on retirement, and after the first extension the paperwork is basically the same, as long as you keep copies of everything. The following years you just need updated bank letters, and new pictures, unless you move, but the big difference is you can legally work with a marriage extension, getting a work permit, which you can not do with a retirement extension. It is illegal to work on a retirement extension. This makes a big difference if you ever want to get permanent residency or citizenship never having to deal with immigration anymore, as you must work for at least three years, and you can never do it with a retirement extension. Being under the legal retirement age in your passport country, the government is going to want to know where the money you live on in Thailand is coming from on a retirement extension, legally, as you will be too young to be getting a pension. Just something to keep in mind.

Can you change reason for extension the first time? Thought you could only change from second extension onwards.

#1 is correct. Once you have your marriage extension in your passport you can spend the 400,000 as you want. You just need to make sure you're at 800k for 2 full months before you apply for your extension. Make sure you know it's 2 months before you apply, not 2 months before your extension ends.

Also you're not changing any visa. You won't have a visa it all. You're changing your reason for extension of stay.