The Lincoln style undercarriage and rudder were on her from day one; look at any photos of her during the period she spent on mapping duties. The same undercarriage and rudders were also fitted on Lancaster B.VII - such as NX611 - and Lancastrians.
Potential fatigue life could be infinite; the structure is relatively simple so you can effectively zero the clock. The Shackleton fleet had a fatigue remediation programme and some aircraft were redone twice - as mentioned earlier it was the spare booms from this that allowed PA474 to be re-boomed in the mid 1990's.
Even so with about 4000 hours flown before the rebuild and now being on a second set of spar booms she's still a way behind the highest houred Lanc, which is Canada's FM212. That aircraft has done around 8069 hours on its original wing!