As long as you find some way to use your English regularly, you shouldn't lose most of your knowledge. Especially since you say you've been using it since you were little. Following an English TV series or picking up an occaisonal book in English would be enough.

Good luck with French!

I understand that can seem a bit tricky, at the same time, French and English share a lot of common vocabulary, so it would be best to embrace this as an advantage. My recommendation is simply that you pay careful attention to the exact pronunciation in both languages, so that you will always be sure which language a given word belongs to.

Hi~ I'm a spanish native speaker that have already learnt english and i'm learning french, right now. I know what you mean~ I'm also a bit scared to forget some of my english skills since my brain is being fed with french vocabulary and grammar. I think a good way to keep our english alive is to write, to have online conversation with friends from webpages like Italki, for example, or maybe to write a diary. In that way english will be always present in our daily life no matter if we're learning other languages :) I hope my suggestion could help you a bit.

Have a nice day!