I did the same a few years ago... except I swapped the whole upper. I went from the Bushy A1 with 11.5"/5.5" bbl to a bushy flat-top with a 14.5" fluted bbl and Phantom supressor. I've been very VERY happy with this move.
I had recently decided to swap over to a RRA 16" mid-length handguard barrel, but I"m still going to use this bushy upper.
For the record, my personal experience has been that the RRA uppers match my pre-ban bushy lower very well. In fact, the 6 year older upper from Bushmaster was a darker black than the lower, and the RRA matched it better.
But I WILL SAY THIS... The 2 armalite Uppers I recently had in my posession were a TERRIBLE fit to my Bushmaster lower. The slop was unbelieveable. The first was a return because it was shipped as a post-ban by mistake, so I hoped the fit was an anomoly too. But the pre-ban replacement was just as sloppy. I sold it to a man who had an Armalite lower, and he says it fit like a charm. Go figure... The RRA uppers I've had experience with fit VERY well on my Bushy.