It's not just 75% - the first phase is 75% for a full difficulty adjustment period.

When they stay at 75% for a full difficulty adjustment period then they will start broadcasting willingness to mine a bigger block.

When enough have signalled intent to accept bigger blocks for a full adjustment period WITHOUT dropping below 75% then they will start mining bigger blocks.

That's what I have seen them posting, so it will take maintaining 75% consensus for several difficult adjustment periods BEFORE the first bigger blog.

The only thing that could throw a wrench in that is UASF in which case they may need to emergency hard fork earlier.