Replace all the resistors with 1% metal film. It will be a hell of a lot more stable and accurate.
Please, a big lordy NO, in this case.
"There are no carbon resistors or paper capacitors."
The Hickok's were designed and built like the proverbial tanks.
If it is in need of cal and after that procedure is figured out, if it goes back into cal without resorting to an unreasonable trimmer setting, like all the way on the pin or something, no need to troubleshoot anything and by all means no need to blanket replace parts in this one.
However, due to the rigors of time, a very wee spritz of Deoxit on just the contacts on those wafer switches is a very good idea. And if you still need to calibrate, another wee bit on the cal pot and mechanically turn it from pin to pin a few times and then set it to cal point.
I've seen several where the only thing needed was the Deoxit on the contacts and they fell back into cal so close as to make me stop right there.