This exchange on Twitter yesterday is perhaps clearer.
Patrick J. @wallyboag 20h20 hours ago
@FrontierCare I have a RT flight booked and would like to change the departing flight to a day earlier. Can I cancel the 1st segment... Cont
Frontier Airlines @FrontierCare 19h19 hours ago
@wallyboag If you change a tix, there is a change fee and fare difference (if any). Our Res Agts can help assist you- 801-401-9000. ^BG
Patrick J. @wallyboag 19h19 hours ago
@FrontierCare Does canceling one leg of a round trip flight count as a change?
Frontier Airlines @FrontierCare 19h19 hours ago
@wallyboag You can cancel one and hold it for the future (90 days) and applicable fees would be applicable for rebooking. ^BG