"When the community accepts an agenda item, it immediately becomes active. Since it was already dormant in the updated code, the vote has the ability to activate it immediately."
In reality, there is a ~1 month lock-in period, to give everyone time to upgrade their software before the new rules activate.
In this article https://coincentral.com/decred-lowdown-decentralized-blockchain-governance/ it is stated that:
In reality, there is a ~1 month lock-in period, to give everyone time to upgrade their software before the new rules activate.