There are a couple processes here. If you want to reassign vm resources to an ISE node, you can do that assuming you shut it down. 2.4 requires vcpu changes regardless if you are looking to use the small or medium templates. So no issue with the resource changes.
The second piece, going from a medium to a small from a licensing perspective. There is no published process for this, and the guide only indicates that Cisco will provides a one to one count of ISE-VM-K9 to ISE-VMM-K9 licences. You can run small vm's on medium licenses, but i'm not sure they will issue ISE-VMS-K9. If you really want small vm licenses instead of medium, you could ask at the contact below. Having the new licenses issued before the upgrade will not cause any problem.
From the Cisco guide.
"If you are planning to upgrade to Release 2.4, contact ise-vm-license@cisco.com with sales order numbers that include vm purchase to procure one medium VM license for each vm previously purchased. You should also include your CCOID along with the sales order number."
