Upgrading Master-Slave MySQL
You cannot switch roles in the middle of an update between major versions. MySQL-5.6 will generate binary log messages that MySQL-5.5 won't understand. So if 5.6 is every a master of a 5.5 slave you can expect replication to break.

At all stages slave version must be >= master version.

So update the slave by one major version 5.5 -> 5.6. Ensure everything is running correctly. Then update the master by one major version. The binary log messages generated with the update will have no effect on the slave already updated.

While it might be tempting to make a new node C and go through convoluted update process your are better of using a test environment, doing a slave update, a master upgrade, start slave (and repeat until at the latest major version you want). Practices a few times. Test your applications against the final version. Schedule a small outage and run through the steps prepared and tested.

Have a fallback plan, i.e package/config/datadir of original saved and ready to put into place). Test your fallback plan.