[morzh]

paper caps: not only they change value (that would not be a problem as they are rarely used where the value is critical, especially in decoupling where only the lower limit is important) but they develop leakage which is why we need to change them. The leakage results in DC current and this affects biases in tubes where they serve as DC blockers, changing the tubes' working regimes, sometimes to the point of them not working at all.
And if such a cap is in decoupling it no longer decouples well plus might heat up and then smoke, possibly also taking the power supply along with it.
So, try your best to reach them, yank them and change them.