Change Pro Sites level price

We are having an issue where if we change the price of a Pro Sites level, then all sites signed up at the previous level stop working.

i.e. these sites just stopped working when it came time for renewal even though payment went through.

As you can imagine we have had some pretty p****d off customers as a result of this.

Has anyone else had anything like this happen before? Is there any issues with altering a sign-up price for a Pro Sites level?

I'm sure I saw somewhere in this forum that this is OK to do this but we are seeing broken sites on our MS network.