I got the new suspension. Here are my experiences with the upgrade:
I ordered the HRX Footers from the Elusive Disc, a reputable retailer. They took a long time to come; availability was subject to VPIs production schedule. Bob at Elusive was good at keeping me abreast of the ETA. Eventually, a box arrived with no instructions. This was apparently the kit for earlier, pre-HRX versions of the TNT, and it included everything I would need for the HRX as well. Each new footer came as three components, a rubber ball, a tower, and a footer with a cup for the ball on its top and a screwjack for making it taller or shorter for leveling the plinth.
The tower is shaped like a cylindrical tube with a dome on one end. The dome is dark grey plastic, although the cylindrical tube is covered by a silvery metal. Inside the tower, at its deep end, is a cup-shaped insert (cupped for the ball, of course). The insert is press fitted or very lightly glued into a cylindrical well drilled into the inside of the plastic dome. A modest tug on the cup shape (using needle-nosed pliers) was sufficient to pull the insert out of the cylindrical well in the plastic dome. After removing this cup-shaped insert, the rest of the tower was discarded. The cup-shape insert is to be installed into the existing HRX tower.
The existing HRX tower is integrally built into the HRX plinth, and there appears to be no way to remove it. Shape-wise, it is very similar to the tower component that came with the new kit it is a cylindrical tube with a plastic dome on one end. In addition, it has a silvery metal surface that covers the plastic dome. There is a small hole drilled at the top of the metal surface, through which extends the air valve nipple for the existing air bladder suspension. The little metal cap that screws onto the nipple is all that holds the air bladder and footer assembly to the existing tower. (Without the metal cap, the footer falls out when the plinth is lifted off its shelf.)
After unscrewing the little metal cap and then removing the old footer, I let the air out and unscrewed and removed the metal valve assembly from the old air bladder (which was still attached to the old footer). The air bladder and the old footer were then discarded. I reinserted the valve stem through the hole in the metal surface and screwed the little metal cap onto it to secure it in place, so the towers appearance was preserved. (Could the metal cap and valve assembly be potential sources of noise/grundge? I haven't heard any so far.) I found that a bent and kinked coat hanger wire was useful for inserting into a hole in the valve assembly bottom to facilitate handling it while doing this.
The inside of the existing tower has a cylindrical well into which the cup-shaped insert can be located. Notably, the well is slightly too big for the insert. I wrapped black electrical tape around the insert three or four times to get a good press fit into the cylindrical well of this old tower. The coat hanger wire was useful for handling the insert as well, making use of a hole drilled through the insert. (Note the well is deep enough to accommodate the valve assembly without interfering with the press-fitted insert.) After that, all that remained was to put the ball on the upward facing cup on top of the new footer and insert this up into the interior of the old tower until the ball was captured by the downward-facing cup-shaped insert at the top.
After all four feet were refitted this way, I taped the footers to the towers so I could lift and replace the turntable back onto its shelf. After removing the tape, I adjusted the height screwjacks in each new footer to make sure the platter was level, using a bubble level.
I find that I am happier with the sound, although it is beyond my perceptivity and vocabulary to describe or quantify exactly how it has changed. Accordingly, I would describe the sound as the same or slightly better than before, without further expansion. For the record, my installation is well-braced in a corner and never had a resonance problem. I noticed a surprising degree of mental relief at never having to worry about bladder inflation again. I am also very pleased from a technical design-aesthetics perspective to be able to level the table reliably. In all, I thought it was a worthwhile upgrade, despite the cost.