I mostly play my wife's Fender classical guitar. When I change the strings, they immediately sound wonderful. But they do not stay in tune. They go a half-tone flat in the first ten minutes. After about a day the stretching is done and they stay in tune.

After about 2 weeks they stretch out in some other dimension, or something, and lose that wonderful timbre. The guitar is a CG-5 but it sounds like a CG-7 for those 2 weeks. Then it gradually degrades to about a CG-1 if there is such a thing. So, the lesson is, when you go to sell your classical guitar, put new strings on it!