The problem will only occur if the client thinks it is talking to CICS TS for z/OS, Version 3.1 (or CICS TS 2.3) but its message is routed to a CICS TS 2.2 region. This will only happen if CICS TS 2.2 and CICS TS for z/OS, Version 3.1 regions are set up as sibling request processors (AORs) in the same logical server. (This is one reason why mixed-level logical servers are not recommended in CICS.) During a "rolling upgrade", the logical server does, of course, contain mixed-level request processors. However, if you follow the steps in Performing a "rolling upgrade", the problem (of a GIOP 1.2 message being received in a CICS TS 2.2 region) will not occur.
During a "rolling upgrade", the logical server does, of course, contain mixed-level regions. However, if you follow the steps in Performing a "rolling upgrade", this problem will not occur.