Rewriterule for directing part of a servers traffic

[RayYates]

I need to redirect a part of a website to a new webserver
where (oldsite.com becomes newsite.com) and this string (Store_Code=abcd) is in the uri.

if Store_Code=abcd
[oldsite.com...]
becomes
[newsite.com...]

None of the Rewriterules I've tried work.

[jdMorgan]

Welcome to WebmasterWorld, RayYates!

Please post your best attempt for discussion.

Thanks,
Jim