Re-tuning a concertina from 'high pitch' down to 'concert pitch' need not adversely affect the voicing of the reeds if done correctly.

Using a suitable needle file, scratches are made along the reed, not across, near to the fixing of the reed to the reedplate, being careful not to stray too close to the edge. This also avoids metal-fatigue problems.

I've been tuning concertinas, melodeons and accordions for many years - you only get real problems if re-tuning a reed by more than a semitone.

The problem most people overlook is the amount by which the reed is lifted away from the plate at its tip - this drastically affects the time taken for the reed to 'speak'. If the reed is parallel to the reedplate, it won't speak at all, or will be very 'lazy'; if lifted too high, the same applies... only experience can find the right balance.