unsound - thank you as always for chiming in. Indeed a replacement driver is really the beginning of a redesign project and not the end. To elaborate: as you say, Jim considered all aspects as a system. Each driver has its Thiele/Small parameters carefully chosen for its tasks, plus its particular resonances and anomalies which he carefully modeled and corrected in the crossover such that the net acoustic roll-off was 6dB/octave with very flat impedance characteristics to realize very flat frequency, phase, time and impedance characteristics. Note that the only way he could get by with necessarily low impedances was by engineering an extremely resistive load. Changing a driver will change all or nearly all those parameters to defeat the subtleties of the design.
All that said, Madisound is a very knowledgeable distributor and they know a lot about their drivers. Rob has worked with them to choose the most appropriate available replacements.
Note also that Morel's inception was to pirate the Dynaudio designs, complete with international lawsuits, etc. So, the Morel replacements are quite likely the closest available replacement for the D28-AF.
Worth mentioning is the D28-AF was used in the CS2, CS3 and CS3.5. That 28mm driver has 20% more radiating area than 25mm domes.
As a generalization, I had hoped to endorse replacements by this time, but haven't gotten there yet. A real product update requires not only new drivers, but redesigned crossover networks. As a DIY, you can get pretty close with what's available, but a professional undertaking requires more diligence.