Hi,
Yeah, but...the shells aren't wood.
They're a composite of thin wood veneers and glue. Radio Frequency cured Urea Formaldehyde (if they had that technology back then) or something else. Just like plywood...it's not going to behave like solid timber.
Sure, solid timber changes. As you know timber's cellular construction means it's like a sponge, sucking in moisture depending on humidity. Air dried timber, no matter how long it's been sitting around is only ever going to get down to a moisture level of about 12-14%...kiln dried starts out at about 7% and and starts to move back up again with the moisture from it's surrounds no matter how it is finished.
You're right, maybe they are getting better with age as the veneers are pretty thick and they are changing for the better. As to how long it takes who knows. They are very inconsistent though aren't they?
The shells are so badly put together...gaps in veneers, heads don't seat on the shell because it's binding on the wrap...edges are all over the place and yet some sound pretty good even with their original battered 40+ year old bearing edges etc.
After restoring a few of these kits, what i notice is you can get a good sound out of the bass drums and floor toms but generally the small toms sound like ass no matter what.