1. You have to decouple all the sup line from main line. There is no supp line for Hotlink postpaid.
2. No downtime, it's just a plan change on their end and you restart your phone to a new Hotlink service.
3. Cooling off period usually refers to a period of time where you are not allowed to switch plan. For example you cannot change from postpaid to prepaid within the first 6 months of using postpaid. However usually change from one postpaid to another postpaid plan is allowed if your line has no contract.

You can call 123 and ask to downgrade Maxis postpaid to Hotlink postpaid, which will take like 2 3 days. They would even try to give you another offer to stay on Maxis postpaid which you can accept or reject. Can also walk in to Maxis centre to do it. However do take note sometimes the staff at Maxis centre not willing to do the downgrade, they will give you 101 reasons.