Depends on what are the changes that need to be done. As far as I know once a plan is allocated, the only "wiggle" room is at the level of related lists: resource allocations and requested allocations, in addition to what actions can be taken on the plan itself: reduce, extend, complete etc.
Within the Allocation Workbench the Resource Manager has from Paris (if I'm not mistaken), quite a few options to manage an allocated resource plan: replace user, shift allocations, move resource plan.
But as far as the main form is concerned with its form sections, even as an admin once you try and update any field, the plan gets Canceled. I suppose that one might be able to customise and bypass this behaviour, though not sure whether this would be recommended..cause basically any changes in the main form are reflected in the child records (requested/resource allocations) which makes sense to be locked once a plan is allocated.
I personally think that if the Allocated plan requires significant changes in how it has been setup, probably is worth cancelling it and re-creating it, however I would advise to first understand what needs changing, perhaps it can be managed using the current/OOTB functionality.
Hope this helps 🙂
