Originally Posted by Andrew Stallard
In this particular case, the battery is third-party in the first place. It's a Panasonic CR1220/FCN.
As long as that is the part being sourced from another supplier (as is the purpose of this thread), then there is no modification involved.
This may not be true, as the medical device manufacturer may perform their own testing of the part to ensure it meets the required specification.

I know of a case where a potentiometer was replaced with one of exactly the same model as the original part (same make and part number) but this caused subsequent errors. It turned out the part supplied by the manufacturer had been selected by testing to meet stricter linearity requirements than the pot manufacturer specifications. In other words, from a batch of 100 pots only half were deemed to meet the required specification. This is a common industry practice.