I’ve done lots of UOM conversions.
And looking at my system I know see I have the same exact problem as you.
A bunch of parts were created as COUNT.EA, when they should have been LENGTH.FT.
So I created a class CNT-OTH to make those parts be in FT. After doing the conversion, I now have no parts of CNT-OTH.EA
But cannot mark that conversion as inactive.
I wonder if a uBAQ could get around it. I was able to clear the Valid setting on Report Styles (in order to hide them) via a uBAQ. The program itself would allow you to clear it, but upon saving, it would just be set again.