In general, the sound from your amp will become dull, less clear and detailed, the volume might drop, and there might be a bit of fuzz or distortion behind the sound. It’s a gradual loss, it won’t happen overnight.

Ever been on a Zoom call and the bandwidth drops so it adapts and uses a lower bit rate? The sound just loses some detail and presence (and the video goes fuzzy). Same sort of thing.

It’s only the output valves that will need periodic replacement, and they are best replaced as a complete set.

When you do this, the amp will immediately sound new again.

The old valves can’t be recycled. Perhaps keep them as spares in case you might accidentally break one, but once they’ve been used up, you can’t get them back to new performance.