However, the most important point is:
I believe that old forum users will not care about this.
Why?
Because if those guys are real constructive users, not pseudo-constructive ones, they will surely earn lots of merits in the future (and actually got hundreds of merits months after the start day of the system).
If months after the start day of merit system, Hero or Ledgendary members can not earn any merit or just very limited merits, what does it mean?
It means that those guys are very lucky ones, who fortunately ranked up to highest ranks in the forum via the old ranking system.
I called they are at somewhat extent, effortlessly high-ranked members.
It's not a bad idea.
If you want to take both earned and allocated merits, weights are essential things.
So, next question, which weights should be allocated (yeah, automatically allocated weights) based on their ranks and their allocated merits?
My suggestion is 0.75(Weighting factor) for earned merits and 0.25(Weighting factor) for initial merits.