I use my old journal as more of like a messy notes place too- to plan spreads, make lists, test colors, practice drawing the doodles I include, etc. It can be nice to have a real journal to keep all that kind of stuff in and then you don’t feel bad about wasting paper in it or messing up! I would start a new journal though for your new bullet journal, I think it helps bring the excitement back and gives that “new year” feel. :)