Budget Revision
A budget revision occurs when an item that was previously finalized (in Budgeted status) needs an adjustment. It can also be used for Not to Exceed and T&M items (i.e., items where there's no bid from an outside party).
The revision could be due to a change in material costs, a shift in client preference, or a number of other reasons.
For example, a client opts for more expensive bathroom fixtures than originally budgeted, but no outside vendors or subcontracts are affected by the change.
Change Order
A change order should be used when the scope of work with an outside vendor or subcontractor changes, so the agreed-upon contract (scope and price) needs to be modified.