I'll be changing out my tank water heater with a tankless, both natural gas. Can I use the existing vent? Right now my tank water heater taps into my furnace vent.

Probably not.

Your tankless unit will likely be power vented, using a fan to push the exhaust outside. Normally these units are vented through PVC. The smaller pipe size is important. The gases are so cool that it is unlikely they would ever make it up through the existing chimney.

Thanks for the help