Our organization was migrated to Office 365 2013 last weekend. We read through all the accompanying information, did the due diligence, and then tested the upgrade on some staging site collections. I would suggest that the introductory email about the migration should have this at the top in large, friendly letters:
NOTE: YOU ARE GOING TO HAVE TO RECREATE ANY CUSTOM BRANDING FROM SCRATCH. CUSTOM MASTER PAGES WILL BE DISABLED AND USELESS, AND THE CSS SELECTOR MODEL HAS CHANGED SIGNIFICANTLY.
Because, of course, the validation wizard has this nice little warning that says "Your custom master page will be replaced with the default. Be sure you have a backup" (suggesting you can just drop it back into place) and comments about 2010 branding in 2013 from Microsoft are all pretty low-key, completely omitting any phrasing like THERE IS NO MIGRATION PATH FROM 2010 TO 2013 FOR CUSTOM BRANDING.
One would almost think that Microsoft is oblivious to how important custom branding is for stakeholders, and how much harm it causes having to tell businesses with no warning they're going to have to pay to recreate their branding on the new system.
Now if you'll excuse me, my Friday is going to suck.
Have a nice weekend,
Philo
Philo Janus, MCP Bridging business & Technology: http://www.saintchad.org/ Telecommuter? http://www.homeofficesurvival.com/ Author: Pro InfoPath 2007 & Pro InfoPath 2010 Pro PerformancePoint 2007 Pro SQL Server Analysis Services 2008 Building Integrated Business Intelligence Solutions