It depends on the wear on your BCG and how the chamber is set in your 2nd upper. Personally, I don't fly by the BCG swapping route. On the other hand, I've bought several uppers w/o BCG and added new name brand BCG bought from other sources w/o any problems. All function and headspace is within tolerances. This is slightly different than what you're considering so I can't say it's good or bad.
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