Planning for decades

The combined work can be shared under GPLv3 only. (The act of running the program, coll. using, is not restricted by the GNU GPL.)

But if your original source code is still intact (and copyright law apparently – IANAL, TINLA – forbids removing copyright notices) within the combined work and a visibly separate entity, you can still take that (possibly patched) file under GPLv2+ as it originally was, because the GNU GPL in no version has the power of actually changing *other* peoples’ copyright licences.