Okay, my Google fu is weak.
Do I need a different FSB when changing over to an A2 upper?
And are the Fulton A2 uppers decent ?

Or an A1 or A2 front sight post change?

Changing over from an A1 or flattop?

If you aren't working on a retro, and don't care about it being correct.

I believe there are front sight posts made for non-marked fsb

What did your fsb come off of? A real "a1" fsb would be for a .625 diameter barrel. An "a2" fsb would be for a .750 barrel. It has to fit the barrel. The a1 sight post is round and has 5 detents, the a2 is square and has 4 detents. They are 100% interchangable.
Now if you had an A4 Fsb it would be "f marked" and it would be slightly higher than a1 or a2 fsbs. As far as I know, no issue with using an A4 Fsb with an a2 upper, but if you go the other way you may need a taller sight post, but probably not even then. Basically I personally have used all combinations of FSBs, uppers and sight posts with no problems.

No, just throwing a A2 upper on a 16 inch flat top upper.  Apparently , that won't hurt my elevation.
Thanks for all the replies gentleman.

Look at your fsb. It most likely has a "F" stamped into it. that would be correct for a flat top. I think the deck where the sight post is is like .030 higher above the bore than any other fsb. So at worst you'll need to screw your sight post down a little.

I have flat top uppers with non F marked fsb (DsArms) and others with F marked fsb. (14.5 and 16").

I also have 2 A2 uppers, one with F marked , one without.  Sighting in has never been an issue. Both are 14.5".