I can't really think of any time appropriate to alter past data apart from when an error has been picked up that did not affect the situation in a major way - currently I am zeroing some starting account balance figures generated automatically by data uploaded before it should have been (a couple of months ago). It was not seen, because the balance was not noted, due to the fact that it should not have even been there - no-one looks for something they do not expect.

In general any 'used' data needs to be corrected rather than amended.