Does the SDLC have mechanisms in the process to address this eventuality?

Discussion post

The text describes the four phase systems development lifecycle process.

Assume that you are in the middle of the systems design phase, and the client and end-users request significant changes to the system development currently underway.

The client notes that standards and market conditions change rapidly so the client is left no choice but to request needed changes.

In the event that such a change in requirements occurs, how would you as an IT professional respond? Does the SDLC have mechanisms in the process to address this eventuality?