As per my original question https://archive.sap.com/discussions/thread/3888922 I have come to now do the upgrade only to find I have an error.
I came to start mount the standby database after installing and patching Oracle 12c but was greeted with
SQL> startup mount ORA-01078: failure in processing system parameters LRM-00109: could not open parameter file '/oracle/SID/121/dbs/initSID.ora'
So I thought ok I'll copy the original one over to that location and try again only to be greeted with another error.
SQL> startup mount ORA-32004: obsolete or deprecated parameter(s) specified for RDBMS instance ORA-00401: the value for parameter compatible is not supported by this release
What do I do?