If I was concerned about the leftover powder being bad, I wouldn't bother to load those last 8-10 rounds......I'd just chuck it and move on. If I wasn't concerned, I'd have absolutely no qualms about mixing it. Period. If the last rounds loaded with the powder, shot the same way the first rounds loaded 7-8 years ago did, then that would tell me there's no or very little deterioration. Mixing that small amount of leftover powder into a new full canister is going to make such an insignificant difference to, it's really a moot point.
This from the Alliant website.....
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modern smokeless powders are basically free from deterioration under proper storage conditions
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What generally makes powder go bad is light and air. One reason we remove unused powder from our powder thrower hoppers and put it back in the original air tight/light proof canister. Another thing is contamination from fumes or introducing foreign matter. One reason loaded ammo last for so many years is because there is little air or light getting to the powder inside the loaded case. Military ammo many times is sealed, both the projectile and the primer against infiltration. Properly store your modern powder and it will last for decades. Store it improperly and it might not last a year.
A few years back during the first big powder shortage, I came across some W231 still in the metal can in a little, out of the way gunshop. It had not been sold like that for decades. I bought it because there was no W231 to be found anywhere and I figured the can itself was a nice decoration, regardless of how the powder inside was. Loaded some ammo with it using the same load data I had used with W231 with recent production dates and the chrony readings were basically identical. This tells me that lot to lot consistency has been very good for a long time and that even decades old powder still performed like new production.
Reloading is not rocket science. Safe handling and use is very basic and uses a lot of common sense. I too will not tell someone else to mix different lots of the same powder, but I will say I do and will continue to do so.