Hi,
Before making any changes to vCenter VM you have to force a switchover between Primary and Secondary server if virtualized vCenter is the Heartbeat primary node.
If you want to decrease RAM you have to shut down your VM and connect directly to host on which your vCenter VM is running via VI Client shut down your guest and change the amount of RAM.
If you are using vCenter Heartbeat mode there is no impact to be afraid.
Without Heartbeat feature only one impact is that you cannot manage your infrastructure and some vCenter features will not work (like DRS etc.) your VMs and hosts will continue to running without any impact.
To suppress any risks with this changes you can optionally create a snapshot of vCenter server VM or take a full backup of it.
Regards,
P.