Since the Falcon Heavy is just essentially 3 Falcon 9s strapped together, would it be possible to reuse them as Falcon 9s with some modification?
The sideboosters can be reused as a F9 1st stage booster, as they are only slightly modified (nosecone & attachement). The reverse was done during the FH Demo Flight: The sideboosters were both flightproven.
The center core however is a little more tricky. It was completly redesigned to handle the additional forces from the side boosters. Since it is attachable to the second stage, it is still possible but it is not very reasonable: The dry weight of the center core is higher and therefore, performance is worse. That wouldn't be an issue on most payloads, but they are also more expensive to make. Therefore they'll be probably only used for FH where they are required. It just wouldn't make sense to use them for a single stick F9.