I'm not 100% sure but I highly believe it's like this:
Overall Leaderboards:
All stats will be here, your current stats will be updated/combined with your new stats here.
New Leaderboards:
Everyone will have reset stats here (seperate leaderboards).
Legacy Leaderboards:
This leaderboard will be another seperate leaderboard where the current stats are frozen and not updated anymore when the new leaderboard is released, however there is still an overall leaderboard on top of this.
With the combination, I'm not sure but I believe the /stats command will show your new stats, possible alongside your legacy leaderboard stats however if you use a different command (lets use /overallstats as an example). If you use the /overallstats command it will basically shows the combined stats (legacy+new stats=overall stats).
This is the idea I've gained from reading things, some may be false information as I said I'm not 100% sure but I believe it's going to be like this or similar to this.