What would be the reason for increased noise ?
a few cm of added copper wire to the already present copper PCB traces running to the front of the PCB ?
The question should be asked as to why wouldn't there be more noise? It'll be more than a few cm as the size of the pots are drastically different. If flush mounted relative to the front of the PCB, you're probably looking at up to 6cm horizontally (tab positions), and then you need to go vertical, with enough slack to install and remove the pot and to not have too much strain on the joints. There's also another solder joint, and it's building an antenna, albeit short. That's not taking into account the choice of a replacement case and where you place the pot on the panel.
If chasing amps that go for the floor measurements, any fraction of a percent is increased noise. Are you saying there will be zero, or a positive, impact? If you've done it and have had zero negative measurable impact, then I'd like to see that, and how it's done. I'd like to understand that part. I'm not talking about discarding the delta in measurement as imperceptible, or falling in the margin of error. An increase is an increase, whether it's perceptible or not. I'd take it that one of the goals of the amp is to hit as low a floor as possible, and air wiring something in will likely go against achieving that goal.
I'd also think crosstalk may be affected as well, but that would fall more heavily into execution, I would think.
I also say that it might not matter and we'd need confirmation.
I would think noise would be increased, if that matters to you.
Can’t say for sure without measuring, but that would be my first thought. Possible, yes.
The most probable conclusion is increased noise. If you have to ask if you can supplant a pot, then I'd wager that you're not the most experienced, and that probability of noise will increase due to probability on execution. PCB layout can also effect results, if given the same parts to build. I don't see why something as coarse as air wiring would not have a measurable impact.
Can you point me to examples where air wiring something in had no impact, or improved measurements? Perhaps my conclusions are incorrect, but I'd like to understand that as well. I'm sure someone has modded it, and re-cased it as that's an inevitable.