The change “under the hood” is significantly larger than the version number suggests. The internal timing has been completely adjusted. In principle, you shouldn’t notice anything, but the goal is, among other things, to be able to work with more and longer DCC packets in a future step. That isn’t possible yet, but the current change is a prerequisite to get there.
For this reason, TM44 1.23 absolutely does not work with older versions of TM44 firmware and also absolutely does not work with current TM-H versions.

One change you might notice is that the drivers on the TM44 can be controlled in two ways when running on pulse width modulation (i.e., “analog operation”): Fast Decay and Slow Decay. Without going into the technical differences, the effect is that Slow Decay shows significantly better slow-running behavior for small scales. It is very clearly noticeable in Z scale and reportedly also with many models in N scale. In H0 scale, I can hardly detect any difference myself. See also this extensive thread
By default, the TM44 1.23 is set to Fast Decay. The reasons are:

• Slow Decay leads to different speed curves for analog rolling stock, and with an upgrade, you would otherwise have to recalibrate everything.
• Due to hardware limitations, the TM-H can only operate in Fast Decay mode (including any future firmware versions of the TM-H).

You can switch the TM44 to Slow Decay by configuring it with DinamoConfig. You will need DinamoConfig 1.41 for this.

TM44 firmware 1.24

The method for reading occupancy detectors has been adjusted. The method in version 1.23 occasionally led to false-positive detections on my demo layout. The process is now less aggressive, reducing the chance of false positives.