Going around the problem you are asking me (which does not let you change the numbers on the runway), there is only one explanation: that you have built the airport buildings, "on the opposite side of the runway".
If it is like this (and it sure is), and would allow you to change the numbers, we would find the airport (and the runway), turned 180º. Quite a problem!
Initially I have never considered this possibility, because I assumed that the system would ask you on which side you build the buildings, at the beginning of the construction.
Anyway, friend Colin, I'm sorry for that mistake. and also not being able to help you. Do not worry about it. Do it when you can and don't modify your plans. I will wait what is necessary, and I will always be doubly grateful to you.
Kind regards: Delfin