Yeah, I'm also continuing with the grind, since the drops are good.
I have a surprisingly big amount of rainbows to be ampied still, so these rewards are really handy.

As for the update notice, on second look it does say that the current rainbow medal exchange lineup will be excluded after the maintenance on 27.01. though it does not specify exactly for what.
As far as I can say, the girls from the next 2 events will still be added to it, but after that it's hard to say.
A bit of a bummer :/ I did manage to hoard 300 just the other day and got myself a PI Yarrow, but I really hope the change they'll make won't be too much of a downgrade.