Convert a Active-Standby to Active-Active Configuration

I'd recommend you do this in two seperate windows from a change perspective.

I'd use the first to create the second traffic group and move any network objects to it only (floating IPs) and get it running on the currently passive device. I'd also take the opportunity improve your exixting HA setup if necessary (using MAC Masquerade etc.). You may get away with zero downtime but I'd plan for some, especially if device group trusts break and need to be recreated. I find that happens often when I'm touching anything to do with HA.

I'd then use the second window to migrate all the local traffic related objects to the new traffic group on the second device and test accordingly. This would also be a good time to test traffic group and device failover. If you script the move of the objects at least you should need too much time.

As ever, test like crazy, read up before you start and ensure you have off-box backups of all configs etc. before you begin. If STP is in the mix, have a good think about that too.