SEO Question when converting to Multisite with Major Homepage Change

We have a client that has previously been in Canada only, but is branching out into the U.S.A.

The domain is currently the Canada site, but the new setup will be multisite structured like this due to some client requirements:

clientdomain.com - Landing page that links to US or Canada. - Current Canada homepage clientdomain.com/ca/ - New Canada Site clientdomain.com/us/ - New USA site

What can I do to minimize the SEO impact on the current Canada site with their page moving to that new location? Would I need to set the new Canada homepage as canonical for the old homepage?