Well no, they can never do a mass transfer, some people simply can't play on PC because they don't have one, any transfer will have to be individually implemented on player request, and I suspect this is part of the problem, how many thousand accounts will apply for transfer on that first day, what about the people at the end of the queue, how long will they need to wait? A more sensible option would be to implement cross play if possible and then let the console players do it themselves by simply stop playing on Consoles.
I am all for full and free transfer from Console to PC, but if you get a large part of the console player base immediately applying that's going to be a problem, and we don't know if cross play will be possible at all.
It might be somewhat difficult but it can be done. The MMORPG Black Desert Online is doing something similar at the moment.