OK I think that the cause of my confusion is the meaning of "set" which I had taken to be the shutter speed the camera shows i.e in that sense the shutter speed is always set. What in fact is being referred to is changing the shutter speed after the shutter has been cocked.
In the event that you believe the shot requires the same speed and the shutter has been cocked but you decide to make a second check and the correct speed has altered then it will clearly require a lens cap. There is no way to uncock the shutter except to fire it, well not with my Agfa anyway.
Part of my original Q still stands. If I choose to change the speed after cocking what change might I expect? If the next speed and correct speed up is 1/50th and I set that on the dial will my speed be somewhere in between1/25th and 150th ormight I get so close to 1/50th that it makes no difference?
That still leaves the damage question. If this happens only rarely how much damage to the speed mechanism is likely?