Main LV to sub main feed cable replacement

The existing cables are over 20 years old and have been subject to degradation due temperature variations through power loading and the local environment, currently the feed cable to FCS sub main panel failed an insulation resistance test with a reading of 800K Ohms, this reading fluctuates between 800k Ohms and 1.2 M Ohms . The pass level is 1 M-Ohm, we are experiencing these readings on testing other main feed cables (chilled water feed cables) (north main feed cable) (BSD main and HVac feed cables) so a programme of replacement is required.