If we ignore consistency with everything we’ve seen thus far about Force ghosts, I think it might be kind of cool to have them sort of oscillate between their young and old selves, not ever fully manifesting as either. I.E., for Obi-Wan, shoot McGregor, make a CGI Guinness, record audio of both McGregor and Stanton-as-Guinness, and then fade back and forth between them, never going more than 85% or so in one direction or the other, and probably favoring the older version just a bit. Sort of like how the TFA Force vision had both Guinness and McGregor, but also visually.

EDIT: I guess I had an old window that I hadn’t refreshed in a while open; I thought we were still talking about Force ghosts. Oh well, imagine this was a page ago.