I've run a script that, among other things, recomputes activity and memberlevel from when I first added the activity hack, except it runs it under the current rule set and not the old one. There have been a few promotions and a few demotions, but overall, most people are the same. You'll discover why I made and ran this script sometime next month, if all goes to plan...
For the average member, it means nothing. Just a few borderline cases where it means anything. Several Senior Members got moved back to members (long time lurkers), and several truely senior Members got the jump earlier than they were expecting--apparently that system wasn't in place back in the very beginning before they quit, but now it is as if it was; SYG for example is now Senior Member (or he was in the simulation, so I'd assume he is now).
Come on now, I've said many times that there are no demotions, and this script runs from the beginning; if you've ever been well above 50% activity, there is no reason to worry.
The way it is supposed to work is that 50% activity indicates a contribution to the forum; it is hardly likely to get to 50% activity without getting noticed, and a user who achived this would probably be recognized by many if they left and came back. 3 months is a true measure of seniority; the senior members after running this script have all been active longer than the members.