You need the modules firmware to be in place to support the Upgrading by Bus and Upgrading by direct connection.
Furthermore to upgrade a module by bus connection, even though that module is already version 6, you need a UCM at version 6 and Comfort at version 6. This is because the UCM needs to pass the information to Comfort to pass the information to the module on the bus
To make it simple we have designated the modules with the required firmware are version 6.
So to answer your question, you have Comfort at 6.xxx and UCM at 5.xxx, you need to upgrade the UCM to 6.xxx and this can only be done by the programming cable
Once the UCM is 6.xxx you can upgrade Comfort to 6.xxx and itself to 6.xxx
To test the upgrade you can upgrade 6.008 with 6.008 and Comfort 6.005 with 6.005 as you may not have another firmware with another version.
It is not going to change the firmware inside but it shows that the firmware upgrade in this way can work