If people switch to english it will be more about your own comfort than theirs, it's done as a courtesy - i don't think anyone would take offense if you kept talking in french and in fact they'd likely encourage you further.

Talking in english is the risky one here, even amongst yourselves. If they can tell you're a tourist you'll get a pass, if they think you're a local you may meet a lot of hostility because "on est au quebek on parle en francais calisse". Usually just staredowns, sometimes getting thrown out of stores, sometimes more the old fashioned in-your-face yelling and chest puffery. But that's regional, if you're like in montreal or the eastern townships there's a lot less of that.