When we made the change away from rank numbers, it was to remove the necessity of all that bookkeeping.
You should already be able to create a rank between any two ranks, regardless of what the numbers of those ranks are behind the scenes.
If it's not possible to convert a rank of one type to a rank of another type, when it's on the border, then that should definitely be made possible.
In general, we don't want the players to have to deal with the numbers at all aside from keeping track of how many ranks of each type are allowed.
