other word that has two versions

Orient/Orientate and/or, Preventive/Preventative, reminds me of two other such words whose usage has at times, confused and annoyed.

1. Legitimate (as verb) versus Legitimise
2. Disputation versus Dispute (as noun)
Edit:
3. Regnant versus Reigning (as adj.)

Can these words be loosely swapped, one for the other or are they situation specific?