I've been trying to write my own Nava firmware for fun and the accents have proven challenging.Here is how I understand what these new boards are changing.
First, some background on a difference between the 909 and the Nava. Each instrument has an accent input which is really the velocity (loudness) of the sound. In the 909, each accent input is connected to a discrete 6-bit DAC. So this means there are 10 DAC circuits just to maintain constant accent voltage levels on all accent inputs in the 909.
The Nava uses a different approach. It has just one DAC (an MCP4822 which has 12-bit resolution) combined with multiplexers and sample&hold circuits. Two multiplexer IC's (IC111 and IC114) are combined into one big 10-way switch between the DAC output and the sample&hold inputs. Each sample&hold circuit consists of an op-amp and a capacitor. So instead of 10 DAC's like in the 909, the Nava has 1 DAC, a 10-way switch (the multiplexers) and 10 sample&hold circuits.
Now in the "previous" Nava 1.0 circuit (before these add-on boards were added), 7 of the required sample&hold circuits were created by adding a capacitor to an existing op-amp. All instruments except the hi-hats and cymbals have op-amp buffers on their accent inputs; the Nava previous design re-used these buffers to implement sample&hold circuits. This is what capacitors like C89 in the bassdrum circuit are for; C89 is combined with the existing input buffer to create a sample&hold.
The sample-based instruments on the other hand do not have op-amps in their accent inputs. So to make a sample&hold circuit for these, the previous Nava circuit has 1 TL074 quad op-amp (IC110, not present in the 909) that provides the three op-amps needed for the three sample&holds of the sample instruments.
My guess is that what changes with these new sample&hold boards is that all 10 sample&holds get dedicated op-amps. This will make the DAC+sample/hold circuit symmetrical in the sense that each accent input uses the same sample&hold circuit, instead of the creative re-purposing of the input buffers of the analog instruments.
