For Lupin, i'm gonna have to split my opinion for both seiyuu. I think Yasuo Yamada's performance didn't change a lot, though in the first series, he sounded a bit more in control; from the second series onwards, there was a bit more variety in his voice. However, in Voyage To Danger, i could definitely hear a negative change, and Dragon of Doom probably contained his weakest performance yet. Of course, this was at the time that Yamada suffered from declining health, so i can excuse this.
As for Kan'ichi Kurita, i don't think he ever really started sounding old. Sure, he didn't quite sound like Lupin the first time around and still had to find the right chord, but after that, he's stayed quite the same.
Jigen's seiyuu has always sounded bad-ass, no matter his seiyuu's age. Though if you compare his performance in the more recent specials to that of the classic TV anime, it does sound slightly different. His seiyuu has been around for the longest time and i respect that. Replacing him would be extremely difficult, but not necessarily impossible.
Makio Inoue's Goemon is a whole 'nother story. I think that around The Columbus Files (or later), his voice already started to wear on.
I thought Eiko Masuyama's Fujiko started sounding old around Alcatraz Connection. Her voice seemed a little strained when she either complained or panicked and sounded more like a tired housewife.
As for Goro Naya's Zenigata, i'm gonna have to say that Seven Days Rhapsody was when it started going downhill. I could definitely hear the rasp in his voice weakened after all those years. I respect his performance, but it's a good thing he was replaced on time.