I bought the 16" rra 9mm Ar complete years ago, can i convert it into a pistol without re- registering the lower?
If i go short barrel , new buffer tube and vertical am i gtg?

Safe answer no you can't , and unless you are talking NFA firearms there is no such thing as registering or registering the receiver.

The ATF has been back and forth with this issue with conflicting rulings that don't make practical sense.  

You can build a pistol or rifle from a stripped receiver.
You can build a rifle out of a pistol and convert it back to a pistol.
You CANNOT build a pistol out of a firearm  "originally assembled or
produced only as a rifle"

And to make it even more confusing, a firearm can be sold as "parts in a kit that were originally designed to be configured as both a pistol and a
rifle" In this case parts can be freely swapped back an forth as long as they are never assembled in a condition with meets NFA criteria of an AOW or SBR.


ETA:  You also cannot put a vertical fore-grip on a pistol.  That makes it fall under the AOW category.

U could buy a new stripped lower and rebuild lower but y do 16" bbl