Nothing in your plan that stands out as unusual. Looks good.
A couple of suggestions for migrating the internal drives to the FAS2040 chassis:
1. Boot into Maintenance Mode and use disk reassign -s <FAS2020 controller ID> -d <FAS2040 controller ID> to do disk assignment and there's no way to confuse disk ownership (not too hard when only 3 drives belong to second controller)
2. While in Maintenance Mode, destroy the mailbox disks. If you don't, ONTAP will complain about unsynchronized NVRAM logs, panic and core dump.
Run these commands on both controllers:
*> storage release disks
*> mailbox destroy local
*> mailbox destroy partner
During boot up you will probably see some error messages about the unsynchronized NVRAM, no root aggregate found because it's marked as foreign, and so on. These are OK. When you finally see some messages where ONTAP assumes NVRAM has been changed, then it will sort everything out and boot up just fine. New cf mailbox disks will be created during boot up and when you enable cf for the first time.