I don't know if it would be retroactive, but I expect so. Probably payouts that occur after the fork will use the new rules. Doing otherwise would complicate the system significantly for a one time event and is probably not a good idea. The fork will be scheduled well in advance so everyone should know what rules will apply to upcoming payouts.

The 15 minutes instead of 30 minutes as well as the term reverse auction refers to the time period that early votes are penalized. Currently it is 30 minutes (100% penalty on immediate votes, 50% penalty at 15 minutes, 0% penalty at 30 minutes) and after the fork it is to be changed to 15 minutes.