"Even with the change locked in, bitcoin won't upgrade overnight for two reasons"
Because of SegWit's two-week "grace period," the change won't "activate" until block 481,824, which is currently projected to happen on August 21.
While the lock-in ensures SegWit will become a part of the network, bitcoin services still need to upgrade their software to support the new style of transactions.
https://www.coindesk.com/segwits-likely-lock-todays-milestone-means-bitcoin/