I worried about giving my DD enough early exposure to my family language. I was really surprised not to find much available until around 2 years old - but having seen the way her language has developed, I think it's because you don't actually need to worry about it too much when they're tiny.
At 9 months, your child won't even be saying single words yet, so there really won't be any issue with the transition at that age.
Around 18 months, they start understanding more and saying single words, which is nice (and of course important) but it's not the main means of communication yet:it's still mainly body language, pointing, gestures (the adult uses gestures alongside the words) .
Language really takes off at about 2, and I'd try to get him/her into French nursery at least part time by then if possible.
By around 2.5 - 3 they start being understood fairly well by people outside the family, so it will start being noticeable in nursery around then, and possibly frustrating for the child. Not a huge problem socially though since children still play side by side at that age, and although they do speak to each other, it isn't really important to them.
By 4, it seems to start being harder, as they expect to be able to communicate with each other as part of their play, and communication with adults is primarily verbal and more abstract.
Something to consider is that English will become your child's minority language, so in fact that's the one you will need to focus on.
Also, the thing that is really important when they're tiny is to speak easily and freely (and correctly!) with them, which is easier for most people in their native language. Given that your child will learn French easily when you move there, it doesn't seem worth sacrificing that.