Is there any known SEO recommendation for migrating a domain with several subdomains to a new domain name?As Google can see subdomains as internal parts of root domains (link), what could be the SEO impact if we only migrate the root domain (www.old.com to www.new.com) and leave the subdomains unmodified (a.old.com, b.old.com, etc...)?Consider the subdomains indexed at Google, with moderated inbound links, and with links to root domain. Is it worth it to migrate all subdomains or by migrating the root domain (with the proper procedure) will be enough? The goal is to keep our website traffic, while risking as little as possible .I couldn't find any article describing this scenario. 
What is the recommended SEO procedure when moving to a site with several subdomains to a new domain?
seo;google;301 redirect;subdomain;migration