Do the Internal Pages Matter?
I’d take a look at their Google Analytics (or other Analytics - they are using Analytics, right?) and see what Organic Search traffic is currently being driven to internal pages. It is possible the the bulk of their traffic is only being driven by their Home Page: this is common if the bulk of their traffic is from Brand terms.
Direct Traffic (URLs typed in from advertising, print material, et al) is also typically to the Home Page: if they advertise using internal URLs, make sure these aren’t lost in the redesign.
If the Home Page is the main driver, and:
- They are maintaining the bulk of the content on the Home Page
- They will still be presenting it in a spider-friendly format (no graphical text, lazy-loaded JS-driven content, etc.)
- They keep other signals intact (same IP, same Domain Name)
- The quality inbound links to the site are to the Home Page rather than Deep Links to internal pages (look a Analytics Referral and Landing Pages details),
then the overall impact may be minimal.
If a significant portion of their Organic Search traffic is driven to internal pages which are being removed/significantly downgraded, then they may suffer a ranking/traffic hit.
Again, Analytics can help gauge the impact of this lost traffic.
Saving Old Linking Info
If the site’s underlying architecture is changing, such that the URL of the Home Page (and perhaps internal pages) are changing, set up “301 - permanent” redirects from the old URL to the new. Google, other search engine spiders, and live users will all be able to follow an old link to the new content.
Don’t use “302 - temporary” redirects, even if they are easier to set up. Even though live users won’t notice the difference, 302’s don’t pass “link juice” and the rankings will suffer.
We Relaunched and Screwed Up - Now What?
You might consider having Google PPC as a backup plan - if the site suffers a dramatic, business-impacting drop in Traffic, a PPC campaign can replace that traffic more quickly than a site rebuild.