What can I do if I cancel IFR when I shouldn't have?
Since you used the term MOA, I assume you're in the US.

Within the FAA ATC software (not sure about DOD's), the only data difference between IFR and VFR Flight Following is the flight plan altitude. For instance, IFR might have a filed altitude of "060" (6000ft) while VFR FF might have a "filed" altitude of "VFR/065" (6500ft). When you cancel IFR, the controller amends your altitude and leaves the rest of the data unchanged. They don't actually delete your flight plan unless you cancel radar service. Therefore, to resume IFR, all they have to do is amend your altitude again.

Obviously, IFR and VFR are very different procedurally. When you canceled IFR, your existing clearance was invalidated, so you'll need to get a new one. However, since they should still have all your previous data available, it'll probably be similar. But it's less work for them than a normal pop-up IFR.