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.
whereby the extra time gives extra oppertunity to get even better than 75% to reduce orphan risks and reduce the nodes left unsynced, by letting the 25% decide to upgrade or be left behind
The only thing that could throw a wrench in that is UASF
which is a fake buzz word for a HBS (Hard because its node and pool enabled. Bilateral Split because its about banning opposition.
which blockstream are stroking the sheep to sleep pretending its "soft"(the S of UASF) but its not soft at all. its just bait and switching with acronyms of fake meaning (node and pool enabled = hard)
in which case they may need to emergency hard fork earlier.
FTFYclarity
soft and hard is simply
soft: pool only vote
hard: nodes and pools vote
below these umbrella terms is what could happen.. in both hard and soft it can either continue as one chain. or bilateral split
softfork: consensus - >94% pools no banning/ignoring of minority. result: small 5% orphan drama then one chain. minority unsynced and dead
softfork: controversial - >50% pools no banning/ignoring of minority. result: long big% orphan drama then one chain. minority unsynced and dead
softfork: bilateral split - intentionally ignoring/banning opposing rules and not including them. result: 2 chains
hardfork: consensus - >94% nodes, then >94% pools no banning/ignoring of minority. result: 5% orphan drama then one chain. minority unsynced / dead
hardfork: controversial - >50% nodes, then >50% pools no banning/ignoring of minority. result: big% orphan drama then one chain. minority unsynced / dead
hardfork: bilateral split (UASF)- intentionally ignoring/banning opposing rules and not including them. result: 2 chains
if blockstream want to ban anything thats not blockstream then they can play on their 26% hashpower(today stats) all they like. and pretend its achieved 100% by just ignoring the other 74% of the network.. they are still going to be on a minority and less height altcoin chain.