Got a new (to me) VPI Aries for Xmas... what’s the arm upgrade path?

I am not sure that you are going to better the arm that you have already, I am a big VPI fan as they do exactly what I want them to do, simplicity at its best so enjoy what you have and milk it for all it's worth and get some laps on that bad boy. Enjoy the music

Isn’t the 3D a drop-in replacement?

Basically as I understand it. Not sure yet whether it’s a special order version because the side weights in the standard version may be too big to fit and bump into the VTA column.

Congrats on a fantastic table. I started out with an Aries 1 and gradually upgraded everything but the plinth. The JMW 10.5 is probably a better bang for the buck than the 3D IMHO. I never fell in love with the 3D. On the other hand the Fatboy was a huge step up that made it hard for me (almost impossible) to go back to the earlier ones. But I think tooblue has it right - the one you have is good and the choice of upgrades depends a lot on the other stuff in your system (you may have bigger bottlenecks elsewhere)

You'll want to get the gimbal arm version of the FatBoy. It is a drop in replacement I believe.  That will improve your bass quite a bit. Unipivot arms are bad news on a number of fronts. Manufacturers like Lyra specifically recommend against them. Makers like Graham and Basis go through all kinds of shenanigans' to stabilize them. The Basis arms are not even unipivots any more. They are bipivots. 

IMO a good, and not very expensive, upgrade is the dual pivot.  Do some research.  Besides other advantages, it makes setting azimuth very easy.

+1 @melm 

Before you make ANY changes, live and get to know what the current configuration sounds like. Later on, you will be able to discern what aspects of the table’s performance you would like to improve upon.   

Went with the Fatboy Unipivot. It's a great match for my Decca cartridge and works well with the Denon 103 too. Figured I would be upgrading there eventually so why not just pull the bandaid off and do it!