How are you going to ensure that none of the changes that were already merged on REL1_24 branch will be lost, and that none of the changes merged on master that weren't meant for 1.24 will be included, and that all of the changes merged on master will still be kept on master after the new branch point?
I don't think trying to do this is worth the hassle for you, the inevitable awful mess it will produce in Gerrit, and the annoyance for everyone else. The two release calendars weren't synchronised and this isn't the best situation, but I suggest we roll with it nevertheless. On the plus side, now we'll have a lot more time to backport necessary changes to REL1_24.