This is exactly what I did for my FLR. I booked my May appointment at the beginning of April and the very next day they announced the price increase.
You are my new hero! My luck has always gone the other way around... For example, last time around, I printed my FLR(M) and they updated the application the very next day.
But it looks like the first Saturday I'll be able to apply (in-person) is May 12th. And 42 days before that is March 24th. So on March 24th, if the 2018 fee increase isn't effective yet, I'll be booking my in-person! And 7 weeks later, we will be
done!