Keep changes small. I think there is a lot of misunderstanding about what a parallel change is and how to approach it, and some confusion when it comes to use it next to feature flags.
You don't need feature flags to do a parallel change, so if you feel you need them, that is a good symptom that you are doing something wrong.
Unlike feature flags that can last in time, parallel changes should be small and never should live more than a working session, the shorter, the better.
Do you have acceptance tests covering the old functionality? Expand it, add unit test if required and contract.
Do you have test, but they should be modified? Extract functionality, add new tests for it, contract and delete duplicated tests.
