milestone is a point were we can check our process 
achivements steps and if are not in proper way throu we 
must here come readjusting goal. every team always go to 
this process planing->devlopment->testing in time sedule. 
if it not happen goto Readjustig goal same work in diffrent 
way

Depending on the situation adjust it

Milestone: Date by which major activities are performed.
Readjusting: If the milestone seems not to be achieved as
targeted or incur a risk, it should be readjusted setting
goals with different approach.

In addition to above mentioned points, During Project 
development process, whenever there is a requirement 
change / requirement correction happens. PM will need to re-
adjust the goal and milestone.

The recognition of an important event within the project,
usually the achievement of a key project deliverable

Re-adjustment in goals and milestones also come from 
factorizing the critical success ratios in a project. This 
is an ongoing process that gets re-visited in a project 
life cycle, especially impacting the development and 
delivery approach. Goals will hardly have a potential 
change, but milestones will be vulnerable to the demand of 
the time, cost, speeded success and most importantly, the 
utilization of the resource pool - both human and non-
human.

The Partial blocks of achievement is a milestone. 
Particulary for projects more than of 6 months period If 
percieved due to constraint -Technology Change , fresh 
demand from customer(changing the SOW),products becoming 
obsolete  there can be mutual negotitation to change the
milestone dates (some times penalty may be leivied) and the 
goal.

Re-adjusting goals can be viewed as a part of change 
management where impact of changes result in change of 
scope of the deliverables or project.
Milestone is used as a checkpoint at logical points in a 
schedule to ascertain completeness of work at that point. 
In Microsoft project a milestone is a zero duration, zero 
resource intentive activity.

"Readjustig goal and Milestone" is a common technique used
in large projects. Here depending upon the requirement we
need to change the Scope of the project resulting the
deliverable vs. time table. Here always we need to keep in
mind about the BAC (budget at completion) of the project.

It is a part of Change management process. Most probably 
one will Readjust the goal only if the scope of the work 
changes and hence accordingly change the milestone.