Unfortunately we won't have as much discussion on this matter as previously, due to the OP being banned. In my opinion, it would be a fair trade-off for hordes to get regular unit upgrades until at least tech 22, at which point, then reforming would be necessary for further advancements. Why tech 22? It seems to be a favourite for major changes, and marks a turning point between historical eras- both admin efficiency and the removal of naval attrition occur, and now client states will occur there too. Paradox seems to like tech 22 for big technological advances, so it would seem fitting to put the last horde units here too, and the time period (turn of the 18th century) more closely aligns with when hordes actually began to decline significantly.