Couple things to note, the way I've got the Gamerscore figures will be slightly different from now on. I've used the historical change figures over the month from TA rather than the exact montly figures. This is mainly due to Halo MCC, as well as other games, having achievements that don't unlock properly.
When things work fine there won't be any difference, but when achievements unlock late they will be in the month they actually unlock rather than the month they were earned (to prevent them from changing previous months figures or being missed completely).
Also, there will be no more backdating scores for new members, that was initially done to get people to move over to the new league, now that this one has a decent number of members it doesn't seem necessary anymore and could even be unfair to older members, in the unlikely event that it caused a brand new member to win just after joining.
TLDR: Monthly GS figures may be slightly different (blame Microsoft's servers) but it will work out in the end + no more backdating for new members.