The answer to your first question is found in the document you shared.
“Removing virtual hardware resources (vCPU, vRAM, vDisk, etc.) is not supported (similar to moving from a more powerful server to a less powerful server, such as MCS 7845 ? MCS 7825). These migrations require backing up the application, reinstalling on a new OVA file, and and restoring the application.”
For your other question, yes you can do that, but it’s not just the memory that is needed to be changed. You’d also need to modify the base speed of the CPUs.