1. The policy is a change of 90+ minutes or a connection which drops below the MCT.

2. That said, DL is good about these things, but you need to make a specific proposal including date, flight numbers and times. You are more likely to get what you ask for if it's reasonable and you make it a pleasant ask.

3. As to fare basis, doesn't really matter as earning is based on fare paid (or not in the case of award, low/non status).