Well, I've not seen this reported before, and it's a pretty aggressive approach, but...
I'm repairing a *very* used HP-67. (Yes, I know... not really a classic, more of a Woodstock.) The key contacts were extremely worn. After a light sanding of the contacts as described elsewhere, the keys remained mushy with little "click". One had even broken from fatigue: I patched it with a little tinned piece from an old badly corroded HP-25 keyboard. Repaired it perfectly.
The scary part: I used a small nylon cable tie and threaded it through each contact. The cable tie acted as a wedge, allowing me to carefully stretch each metal arch away from the board. I put a little more force into it than I was comfortable with, but none broke.
Guess what? It worked pretty well! The keys have a much more positive click to them, and there are no bounces. It's really not too bad now.
I suggest this as a radical approach to help only nearly expired keyboards. I couldn't bear to see a good machine die.
Michael