Just a point on giving up validations above MUR. If you work at TC (as I do) and you decide that, for example, you no longer wish to hold TC North and Capital and wish to revert to MUR, management would surely have to accept your wish. However operational reasons may dictate that your Capital validation is more vital and would rather you drop North. This would mean that you would now be below MUR and required to train on another sector to get back to the MUR you have to hold.
I know this won't always be the case, but I do think it is something management may do, and I think it's something that they would legally able to do? After all, it's their job to get the correct validations in the room as far as possible.