BS definition:
Bulletin: A modification to the contract documents that is made after bidding closes. It’s issued to the general contractor for review, and may result in changes to the project cost or schedule.
From the AHPP:
Bulletin: A document issued by the architect to the contractor after contract award that may include drawings and other information used to solicit a proposal for a change in the work. Also called Proposal request or Request for Change:
Change Order: An alteration to the Work that results in Scope / schedule / cost change. Amends CDs.
Sounds like a Bulletin has the potential to become a Change Order if the Contractor responds by indicating that the proposed change will affect the project scope.
Is that the basic gist of the difference between a Bulletin and a Change Order?