There is obviously an answer to the question of converting GPencil from 2.79 to 2.8 here. Convert old GP to new GP
And this works but only if you open a 2.79 file with Gpencil in it. The Gpencil shows as annotations and you can use the same datablock on a Gpencil object.
However if you create an annotation inside of 2.8 you can't use the same datablock for the Gpencil.
I don't know if this is a bug or intended but it sucks :-)
Does anybody know of this issue and can help me report it to a dev? Or is there a solution?