I'll start by asking Why and was it sold/bought with the intention of doing the migration this way?
As a wiseman told me (several times). "Honey badger don't care". It would (probably) work at an ONTAP level, but... I think you could run in to some unforeseen issues either during the swap or down the road. There's no way to do the "headswap" online that's for sure.
You would be better off doing the vol move method of migration. and then decomming the 8020 and attaching those shelves to the new 2720.
Here's a link to the controller upgrade guide. I don't believe your exact question is covered.