As regards the illness it's the same thing, if you go to increase existing cover it will be underwritten again as a new policy so the illness will come into play.
Might be a case that you will need to hold onto the old policy depending obviously on the illness and it's effect on cost of life cover. Apply for a new one but keep the old one until you are sure you can get what you want at an affordable price, if there are difficulties there then you can look into getting a waiver from the bank while continuing to keep the old policy if more practical.