Switching from speaking English at home to Japanese?

For anyone who lives with a Japanese person, have any of you switched from majority English at home to majority Japanese, to help your fluency?

I think if we'd started by speaking Japanese at home it would have been a lot easier. But after living together for 9 years, every attempt we make to change our home language seems to fail.

What strategies did you use? Cold-turkey, or gradual? What were the challenges and how did you overcome them?