Because Obi-Wan and Yoda maintained their basic forms upon death doesn't mean that this an absolute rule. There is nothing that indicates that it is the only way their ghosts could have appeared. You would never assume that because all the dogs you've seen in your lifetime are cocker spaniels then all dogs must be cocker spaniels. Similarly you can't assume that all Force ghosts take on the appearance of what the Jedi looked like before he died because Yoda and Obi-Wan do.
One way I think it could have been done is to show all three ghosts as they appear in the original version. Cut to Luke looking at them. Cut back and all three are their younger selves.