Not all of British Columbia can make the change. BC's northeast and much of the Columbia-Kootenays are presently on Mountain time, which means that the Province of Alberta holds the choice of when/whether their own and those BC areas go to a permanent time. Then AB would have to sync with Saskatchewan along their borders, but SK is already on a permanent time zone system. Decisions, decisions.
The northeast is already on permanent PDT.
On further reading, the northeast of British Columbia is on permanent Mountain Standard Time, as seen here:
https://www.timeanddate.com/time/zone/canada/chetwynd
Parts of the Columbia-Kootenays change between Mountain Standard and Mountain Daylight time:
https://www.timeanddate.com/time/zone/canada/cranbrook
while the Town Of Creston is permanently on Mountain Standard Time.