Legacy plan overage charges, want to switch to unlimited

I’m on an older Gigabit plan with a 1.2 TB data limit and have been charged overage fees on my last few bills ($20, then $80, with more pending). The current 1 Gig plan with unlimited data is cheaper than what I’m paying now. I’d like to switch to an unlimited plan before October 31 and request a goodwill credit for the overage charges. Can someone from Xfinity Support help?