I haven't worn my glasses in a like a year and put them on today. I noticed that vision in one eye is slightly "off" as in, the glasses rated -2.5 power aren't correcting it fully. The difference isn't staggering but it's definitely noticeable. Is it possible to increase the clarity of my vision by wearing my glasses and doing eye exercises so that I can get back to -2.5 power from whatever it's at right now (possible a little more)? I don't have any knowledge of optometry so apologies if this is a blatantly stupid question.
I've changed eyeglass prescriptions in the past, which is equivalent to what you're doing. Your eyes are no longer currently accustomed to having that kind of lens in front of them, so it feels the same way it did when you put glasses on for the first time.
I would guess that the -2.5 power is the lens which makes a more significant correction to the lens of one of your eyes. I don't remember the numbers for the lenses of my glasses, but my left eye is further off (more nearsighted) than my right eye, so if I were to not wear glasses for a few days, my left eye would notice it more when I put my glasses back on.
I think it was after I had not worn glasses for a while - maybe months - that my vision was evaluated again, and both eyes had changed. My left eye was less nearsighted and my right eye was more nearsighted. So, I would guess that not wearing glasses makes your eyes exercise harder at focusing, which might change the shape of the lenses in either direction.
Maybe after wearing your old glasses for a couple weeks, giving your eyes a chance to get used to them again, if that eye is still not focusing satisfactorily, you should go to the optometrist and have them see if your eyes have changed. (Two weeks is a rough estimate, but I think it's a good amount of time.)