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If consolidating the domains into subdomains is inevitable, then 301s are going to be the way to go. Just be careful to redirect everything to where it should go in the subdomain, and do it one chunk if you can. You don't want to 301 some stuff, then re-301 or do a second round later. Be as clear as you can to Google. once you are ready to move the site, do it all at once, so Google can clearly see that everything from the domain is now at the new domain. So yeah, wholesale switch, just make sure the client understands what he's getting himself into. There is no exact number with regards to traffic loss, chance of recovery, risk of things not going smoothly, etc. Make sure they are aware of all the risks and how to best lower the possibility of something bad happening.