The only way to do plan switching in CICS is to do a CICS start on tran B. Obviously that is not a good idea (for reasons we will not discuss here).
That is why packages were developed starting in DB2 version 3. You use packages for all your CICS programs in an application and make sure they all belong to the same plan. Obviously, not every CICS program package for the entire shop should be in the same plan (such as when other applications exist). Any time you do need to switch plans, you just start that tranid, instead of xctl.
You should look at the RCT tables and find out more information about the DB2 CICS attach in the manuals.