Flush trim bits are rarely flush to begin with.
If they are, they might cut the template!
It doesn't hurt to get a re-grind and if there's enough carbide, it can be done several times.
Notwithstanding, the regrind is never as good as the OEM grind.
As a rule, only the face of the flute is touched. The OEM hit, maybe hit 4-6 other surfaces the re-grinder would never do that.
So in my case, once they're dead, I scrap them.
Cutters are so cheap these days, it doesn't pay to have them sharpened!
And, yes, there is less diameter, but not much.