Having two sets of rank badges with different colours coexisting, with different semantics that depend on the set of rules that were in state at the time would seem a bit weird in my opinion, and dealing with the overlaps in non-trivial. Take for example a Legendary who happened to earn enough merits to become a so called self-made Hero. Which of the two sets of forum rank badges should he be wearing?

I would favour though displaying the number of earned merits alongside the merit counter (i.e. 120/ 620 -> 120 earned merits out of a total of 620 merits).
A simplified version of rank badge change would be to add a trailing (or leading) icon with an enclosed number, equivalent to target earned merit values (i.e. 100, 250, 500, 1k, 2k).