Maximus then you mean first key is not voided?
No the first key is not voided. The change key can only be used to set the lock to accept any one of up to 7 different keys. The change key by itself cannot open the lock. There is no need to set the keys sequentially you could start of with key No 5, change to No 2 then No 7 or any other pattern you liked.
The uses I see are
1). when you only have 1 key and the change key it can be used so that both keys and presumably both key holders have to be physically present to unlock the belt.
2). When several people have a legitimate right to open the belt they could all hold keys and the belt could be programmed to that key before the wearer went there. Only examples I can think of would be the family doctor if they understood and accepted chastity, relatives if going to stay long term, there are probably others I can't think of.