My goodness your going to take the BP pellet out because you heard it could downgrade / damage the brass?
I would be doing that to smokeless rounds long before I touched any of the few BP rounds I have.
The BP has been in there for perhaps 100 years, if they haven’t downgraded by now…
I have a lot of US, B.P. sporting rounds from the 1860’s - 70’s & the same in British sporting / military & not a problem with any of them.
Humidity is not good, so perhaps reducing that exposure would ease your mind.
I’ve heard any & all MK 1 powder rounds need to be in the deCoux flea-O-flea collection. So please send-em over Richard.