Just to be clear, does your query relate to Mortgage Protection insurance (i.e. the policy that will clear the mortgage if one/other person dies), and not home insurance (the policy that covers the house against fire/flood/burglary)?
You can change mortgage protection policy after the mortgage has been drawn down.
If you have any doubts about the medical issues, just tell the insurance company what you know, and they will take it from there. It's not worth losing sleep over possible voiding of the policy.