What is a word that encompasses both "extend" and "shrink"

Solution 1:

Strictly speaking, neither extend nor shrink should be used with deadline in the first place. (Although I understand the idiomatic use.) A deadline is something that happens at a fixed point in time, it doesn't have a duration. Instead, you extend or shrink the duration of a project or the period of time before a deadline is to occur. (A generic word that applies to both extending and shrinking is resizing—but that has more of a spatial sense than a temporal one.)

For deadlines, you would reschedule them:

: to schedule or plan again according to a different timetable
especially : to defer required payment of (a debt or loan)

// She called to reschedule her appointment.
// The meeting was rescheduled for Tuesday.

You can also simply say that you move deadlines.