The official communication says they are just fixing the score displayed on the fight map screen:
So, from all we know the switch to an individual leaderboard is a bug that got introduced as part of fixing the display, and should be corrected soon. Anything else would be very foul play, unless they plan to refund shop tier purchases. Some guilds went all in to compete for the top ranks, yanking the grand prize away a day later, after participation fees have been paid, would be unlikely to go down well.
I’ve opened a bug report to address the currently incorrect leaderboard type.