I have only one lower, which I use interchangeably with three uppers I have bought/built. When I obtain/produce an upper, I include its own dedicated BCG with each upper, so issues of this sort never arise. I think it's well worth the additional investment.
Since I now have another shooter in the household, it's probably time to obtain another lower. When I do, I'll be building another 24' bull upper for Varmint/Target shooting alongside my Granddaughter.
The other two existing uppers are 16", one a bull barrel with a BDC scope, and the other a lightweight run/gun with a Vortex SPARC.
Greg