I purchased several 5 gallon buckets of exterior satin latex to paint my
wood siding house, after putting some on, I now realize I should have used
flat since the wood is original (70~ years old) and the satin seems to
highlight every imperfection in the siding.

I inquired with several paint stores about making the paint flat, and they
said it could not be done. I noticed some talk in here recently about making
gloss stain flat using some sort of flattening agent, would something like
this also work with paint?

I was considering just purchasing new paint, but if I could make the paint I
already have flat(er), it would obviously save me a lot of money.

Any advice is appreciated!