Fix/redo grout between ceramic tile and wood laminate before putting down threshold (transition)?
I don't see any reason to redo the grout unless the gap would be large enough to leave your transition unsupported in the center enough that it could crack if stepped on, which doesn't sound like the case.  I would however clean out the old grout.  If you don't it will undoubtedly continue to crack and crumble and you'd be surprised at how annoying a sound it can make when someone steps on or near it and the floor flexes.  Personally, I'd try cleaning it out thoroughly and caulking the gap. I'd also suggest silicone caulk over Liquid Nails for "gluing" it down as well.