Like a lot of you, I've been shooting many old bows for decades now, along with a few new customs here and there. My favorite old ones are Widows and Wings from the 60s, and Kodiaks from '54-'64. When I began shooting many of them again last year after a long layoff, I calibrated my old scale and have weighed at least 75 old bows in the last few months. And many years ago, I scaled many of them when they came into my collection. Have no definitive record to rely on, but-with a few exceptions-almost all of them were within a couple of pounds of the weight the bowyer put on them. Most were at least 20 years old when I got them, and most are 40 or more now. They're not all strung when stored, of course, but I leave my favorite few strung pretty much over the entire year in which I intend to hunt with them.
Nothing scientific in this opinion, but my measurements don't support the draw weight changing over time(seems like a limb would have to literally be in the act of failing for it to weaken....and I've never seen a hint of that. Also, you would think it would then eventually catastrophically fail....but they just never do!
Might be more scientific if a bow were chronographed with the same setup over a long time....but, who would do that? Not me!
I've got several fine hunting weight BWs from the '60s that still perform right with some of today's real expensive customs. And they scale almost exactly what the Wilson Bros wrote on the riser.
If the not overheated or physically stressed, and the wood remains sealed, so moisture can't affect it, I think old bows will remain unchanged indefinitely.
Now....if you just want a new bow, that's maybe another story you can put together to get it past the little woman!