It would be change fee only, no fare difference for day of travel changes, at airport; I have called to clarify. It would be subject to availability of course.

As per the WestJet web page: ....."10 Changing to an earlier flight, or re-accommodation for a missed flight, to the same origin and destination on the same day of travel must be requested at the time of check in (or at the airport within two hours of a missed flight) and is subject to availability. Difference in fare will not be charged or refunded for a change to an earlier or missed flight....."

Thanks for pointing out the possibility though.