Originally Posted by ernestnywang

If you are changing from X to O, in the vast majority of cases additional taxes need to be collected, and the only way to do this is to re-issue. If an agent does not charge you the additional tax you owe, that's a clear error.

If you are changing from O to X, at the airline's / agent's discretion, it may be possible to leave it as an O even though it is under 24 hours. This is doable.