What happens to the plot thread where Goku's grandfather died when Goku
transformed after seeing the moon, and long before Bulma came on the scene. 

Hmmm, try this. Ranma must have some pretty good senses, and sees Bulma
coming, looking for the dragon balls.

He decides that Goku needs to get out into the world. He uses Shampoo's
shampoo trick to change Goku's memory, and make him believe that his
grandfather died. Ranma could then re-enter the story as the Turtle Hermit,
which is a guise he uses occasionally to check out the world, and what has
happened in the last century or so. 

Just a thought, or you could simply proceed with it as is, with Goku's
grandfather [Ranma] still being alive. That would affect matters quite a
bit I would think, and change a great deal of the original story.
Which is not a bad thing. 

T.H.