Yes, in some cases proctitis can become left-sided or even pancolitis. Some people report this has happened after they discontinued medications when they reached remission, but I'm sure there are others in whom it spread even though they took the meds as prescribed.
The inflammation of UC in any flare almost always starts in the rectum and spreads upward. So people who have had pancolitis diagnosed with previous flares could show only proctitis on the next flare, especially if they treat early and effectively to prevent the inflammation from getting worse and becoming generalized. This why you will see so many people here who start or ramp up rectal meds whenever they feel they may be starting to flare.
On the other hand, there are those of us who have proctitis or left-sided UC who have horrific flares and yet the inflammation never spreads beyond the rectum or descending colon. Why? I'm not sure anyone knows that.