It makes no sense to me, you can cancel the life assurance if you want or is it assigned to the lender or something?
To the best of my knowledge you can take out a new joint decreasing policy which is the same thing as mortgage protection.
Maybe one of the insurance people here will confirm if that is correct, always thought you could!
Possibly try some of the online insurance quotes to see what prices you get, just for comparison between the two types of policies.