Not true - a MEC covers you for all prescriptions, irrespective of whether they are related to the condition you got the certificate for or not.
Coming back to my earlier post I've found my covering letter and it clearly states you can continue to use it until it expires, even if your condition changes. The only restriction is you can only renew it if you are still on meds
The exact wording on the back of the letter is as follows:
Edit to add - I've added a scan of the back of the covering letter in case anyone finds it useful (click on the thumbnail to view)
View attachment 13337