BIG-IP LTM VE virtual hardware upgrade: Migrate to new virtual appliance or upgrade in place?

We have 2 instances of BIG-IP LTM VE deployed in separate environments and both were deployed to their respective vSphere environments several years ago and are still running at VMware HW Version 7, though the BIG-IP LTM versions running on them are currently 14.1.6. We have been upgrading them by ISO files (standard software upgrade procedure) without changing the VMs' configurations or virtual hardware versions. We have since upgraded our vSphere environments to 6.7 U3, and we plan to upgrade the BIG-IP LTM VE appliances we have to version 15.1.x from 14.1.x (and later, our physical F5 BIG-IP nodes from 13.1.x to 15.1.x).

What is the best approach or path to upgrading both the virtual hardware and software versions running on them? Do I just upgrade the virtual HW in place to the latest available, or is it better/easier to deploy the 15.1.x VE OVA and just backup/copy/migrate the configs (UCS..?) over to the new appliance? If upgrading HW in place, does the VM need to be powered off? I want the most stable upgrade method possible, since the HW upgrade is a big jump up, especially when also upgrading a major OS version. Note, we can easily power the virtual F5 appliances down as these are in test environments.