I RECENTLY REPLACED A BUSHMASTER UPPER RECEIVER WITH A COMPLETE BCM UPPER GROUP. I AM WONDERING IF I SHOULD REPLACE THE LOWER WITH A BCM LOWER. WHAT IS THE DIFFERENCR ?
First, take your caps lock off, it’s bad internet form.
Second, is the Bushmaster lower causing issues? If not then I don’t see a reason to spend the time, money and effort to replace it. Yes it’s nice having a matching upper and lower and there is no doubt the BCM lowers are top-tier, one of the best available, lowers on the market today but functionally it won’t make a difference.
I think its really your call, should be fine though. Generally there isn’t a lot of difference in lowers. But I’m sure someone more knowledgeable will be along shortly.
I’m running a Colt upper on a Rock River Lower. I don’t feel the need to buy a Colt Lower for it. 
Unless it bothers you having different brand uppers and lowers there is no reason to change. You solved all of the issues bushmasters have by replacing the upper. The only thing you need to check is to make sure the receiver end plate is staked and you are good to go. I am running a colt upper on a bushy lower and if I did not have a department issue carbine, I would have no reservations taking my personal gun to work.
Thanks for the replys. Sorry for the capital letters.
The lower is essentially just a shell that holds all of the important pieces in place. As long as the dimensions are correct (pin locations, mag well cuts, etc) you should be fine. For the most part, Bushmaster is sending out okay lowers.
I was the opposite:D I have a Colt Upper on a Double star Lower and Just had to Have a Colt Lower .The DS Lower is fine ,But I just felt Like, I needed the Colt lower to make My weapon All Colt. Plus Now I have a lower for a new Build:D