Issue
You are re-branding your domain and would like to know if it is possible to transfer cookies from one domain to another.
Solution
While there are general approaches to sharing cookies across domains, this is outside the scope of Marketo Support. Included below are some non-Marketo resources on approaches for sharing cookies
https://stackoverflow.com/questions/12370495/share-a-cookie-between-two-websites
codeguru.com/csharp/csharp/cs_internet/article.php/c19417/Sharing-Cookies-Across-Domains.htm.
The best point of contact will be your web developer and they will be able to help you out on this regard.
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