All of the old MR2 fuselages are extensively examined for corrosion and fatigue, etc. There are areas of it that were expected, at the outset, to need replacing. There have been few big surprises. I believe that the pre-transformation condition of the fuselage shells are one of the (few?) success stories in the MRA4 air vehicle programme. Although the Nimrod fleet might be 35 years old the aircraft have each only flown an average of 17,000 hours, which is less than 10 hours per week, which is a ridiculously low flying rate. Even then, its pressurized for less than half its flying time (although lately, the shell has been pressurized more often).
Regards
Ed Set