The problem you're talking about doesn't exist and hasn't existed for a long time. There's nothing illegal about reverting a gun built off an "other" to a pistol. The whole point of them changing the designation was to remove the ambiguity so that if somebody did build a pistol or rifle it didn't matter. An AR lower other is almost a handgun for ATF purposes. You must be 21 to buy an Other. Once it crossed the counter as an other it's basically bisexual from birth at that point.
it essentially eliminates the confusion.
This "problem" was only ever a problem when a bare or partial lower could originate as a title 1 rifle and be legally sold to 18 year olds. That hasn't been the case for well over a decade, maybe 20 yrs at this point? Maybe way longer. There was a time when you would buy lowers that were demarcated as a pistol lower and you would have to be 21 plus because it would effectively cross the counter as a handgun. Nearly everything else was defaulted to being declared a rifle regardless of its state of existence. Those days are ancient history.
Bottom line is that most humans who have a lower who crossed the counter as an "other" effectively have something that they are free to convert back and forth as they wish. The feds are not coming to get you. (They never gave a shit about this anyways outside of ffl operations anyways. )