As long as there are suitable integration and system tests, this is no problem at all. Bonus if there are defensive assertions as recommended in the paper (desi

As long as there are suitable integration and system tests, this is no problem at all. Bonus if there are defensive assertions as recommended in the paper (design-by-contracts style). Sometimes it is easier to refactor a system without lots of unit-tests, as you need to update/fix lots of tightly coupled tests. What matters is what the system does (as seen from the outside), not how the underlying classes/functions work. They are mostly incidental.