Re-baseline a part of a plan?

Thanks Sam, but that's not what I'm asking.  Of course work items can be changed.  But can a small set of work items on a plan be re-baselined, recalculating their Planned start and Planned End date while the other work items on the plan maintain their original baselined planned dates? 

Just to clarify further, Planned Start Date and Planned End Date are attributes from the Formal Project Mgmt process template. 

again,as far as I know, any workitem can be changed at any time by someone with the right authority. so YES, you can change the planned start and end dates of any workitem at any time.

policy determines if you SHOULD, not if you CAN.  Baselining is a policy process.

The Planned Start Date and Planned End Date are read-only fields by default in the FPM process template.  They are set when a plan snapshot of type baseline is created.  I'm rather not make these fields editable.  Perhaps if I filtered the plan view to the few items I want to baseline and resnap the baseline that could work, but I think you can only have one baseline snapshot at a time per plan.