I think it might depend on how flexible you are on basing, if you're happy to go anywhere you will probably get an earlier start date. If you absolutely want a particular base it may take a bit longer. Of course it also dpends on you being rated / non rated and on training capacity. In my experience being flexible on basing can be an advantage if this is a company you want to join. The company are very good at getting new joiners to the base of their choice.
As regards the command upgrade process, I can only speak for the Airbus fleet but I don't see any of the issues mentioned above. I don't have any experience of the Boeing side of things here.