If you are asking if you can restrict access to edit a field within Project
Professional once the PM has the project open in edit mode then the answer
is no.
Out-of-the-box, you cannot limit the access that a user has to specific
fields once have given them rights to open and edit the project schedule.
The only way I can think of that you could get around this is to create a
macro that will prompt the user for a password if they attempt to make a
change to this particular field. Now this could get tricky since you might
not be able to capture the field change event. But you could do it by
copying the value into a temporary field on the project open event and
compare the value at the save event. If the values are different than you
can prompt for a PMO password to save the plan as is or force the user to
cancel out of that prompt and instruct the user to change the value back
before they can save the plan.
Try to post this macro request to the microsoft.public.project.developer
group if you don't know how to write this kind of macro and don't have any
developers of your own onsite.
Jonathan Sofer