This is something people have asked me in real life recently, and it was in the comments here, too:
Have you been able to successfully switch which language you speak with someone? How? Do you have any tips on how to do that? Especially, how would you change the main language in your home?
Personally, I've found that what I start with, is what sticks, and that's not always good. We speak Russian with our children, but we've done that since they were born. We have been able to bring in English in certain contexts, but it's harder than I would have guessed. (They're bilingual, but they don't like to speak English with us.)