The posts about the black powder substitutes being a waste of your time in the flintlock are absolutely correct. Black Powder ignites at around the same temp that paper ignites, 451 F. The modern replacements don't ignite until well over 700 degrees. I have actually gotten Triple 7 to ignite, but we had to fill the touch hole with powder to get a fuse to burn into the main charge. The hangfire was epic.
If you are going to test fire the powder, I can honestly say I have never heard of the powder gaining additional power, just losing power. I doubt there is any problem shooting this powder if it has been stored where it would stay dry.
Now, on the other hand if it has gotten damp and has clumped together, you got a problem. By breaking up the clumps you will get very uneven sized granulation. The finer the granulation the more power it will produce in the barrel. A recent Muzzleloader Magazine article showed a percussion gun with the nipple blown out from using re-granulated powder. If you are bent on saving the clumped powder, get a copy of Dixie Gunworks catalog and look up the screening sizes for the different granulations. That way you can seperate out the different grades from cannon powder to priming powder.
Side Note: Powder is generally sold as Fg, FFg, FFFg, and FFFFg. DuPont was the company that started this gradation chart, F standing for "Fine" the lower case g for "grade". The more F's the smaller the grains. DuPont worked for Lavoisier who later identified the element, oxygen. And it was DuPont's factories that were built with thick stone walls on three sides and flimsy wooden walls on the side facing the river so that if it blew, they could be back up and running in days. DuPont offered free schooling and housing for the widows and orphans of his factoried.