What is the recommended SEO procedure when moving to a site with several subdomains to a new domain?

Question

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.

Answer 1

Just redirect all of them to new address, and use Change of address tool from Google Webmaster Tool properly.

Now, traffic is depend on your website reputation. You can check out your crawl stats in Google search console, if It is high, then Google will update your all old URL to new one within few weeks, otherwise it's take some time to recover all of the traffic.

Also edit your internal webpage, and point to your new source, don't use 301 redirect everywhere, I mean if your new page still point to old one, but you redirect them to new one, then it is good to edit all of the internal-link yourself and point to new domain articles. Here you can use Google Webmaster Tools to identify your all internal links( Just go to GWT > Search Traffic > Internal Links).

Also try to contact other webmaster, who links to you, if they also change your old domain to new one, then It will good for you, to recover most of traffic. Here you can use GWT to checkout who links to you (Just go to GWT > Search Traffic > Links to your site).

I have also migrated my new site (not having much reputation aka PageRank), and it's take 2 months to recover my position back in search result.