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Thanks, William. All valid points and questions to raise. I especially appreciate the estimated loss in traffic. That was a number I couldn't put my finger on, having only switched domains for sites that did really poorly in search to ones that had a better overall structure. (Nowhere to go but up - a situation not relate-able to this one.) I can at least give them an estimate of potential traffic loss based on your experience.

Why do any of this? Our sites are mostly ad supported, and the main reason behind consolidation is that the big domain is more valuable to advertisers than the smaller sites. Bringing them all under one global header increases the association between content, and therefore offers more value to advertisers. The main reason for collapsing all domains into subdomains is the ability to measure traffic and accurately follow activity from one site to another - 3rd party vs 1st party cookies. Unfortunately, I don't think that we'll be able to use a folder structure at this point.

Because of the reporting issues, I believe that they will eventually collapse the sites, no matter what I convince them to do right now. Moving into this new CMS requires across-the-board URL changes, even if we don't switch domains. Part of me feels as if it would be better to go ahead with the wholesale switch now instead of risking a second round of 301 redirects in the next 12-18 months. But signing off on the plan makes me really nervous.

I know that questions about this topic have been asked before, but I didn't really find an answer that I could apply to our situation. We have several websites that now exist on separate domains, even though their topics are closely related. We are moving each of these sites into a new CMS and are considering collapsing all of the domains into a sub-domain structure around the strongest domain. Important to note: All of the current domains have existed for many years and have strong site authority, and regardless of the domain decision, in this restructuring we will be bringing them all under a global header. I know that there are SEO risks to moving a site from an established domain to a new one, even with 301 redirects in place, but the team in charge of this move wants to know how much of a hit we would take and how quickly natural search traffic might recover. Maybe and mights aren't really satisfying their questions... Does anyone have experience with collapsing domains into a sub-domain structure and feel like sharing your results? Most importantly, was it worth it???? Thanks.