Domains re-arranged questions

Thank you for helping me to figure it out this.

I have two sites:

The Chinese site is using the main domain as www
The English site is using the sub domain as en

The Chinese site was built first and keywords well ranked in google. But now, I believe the English site will have more potential. so I wanna use the main domain for the English site, and use sub-domain cn for the Chinese site.

I know if I do so. I may lose all the ranking in google. Any ideas to avoid or minimize the loss?

Edited out my original reply as it is invalid now that OP has added more details to his post.