Upgrade to Guardium 12.0 is only supported from Guardium systems that are running on version 11.5 and up.
See these release notes to take into consideration many other factors that you'll need to consider: https://www.ibm.com/support/pages/node/7038757
STAPs are backward compatible, but you'll want to include patching them to the respective version that matches the appliance version, after your appliances are upgraded.