Hello!

Just to be sure... when upgrading Exchange 2013 to latest CU during coexistance with Exchange 2010... does the 2013 upgrade affect 2010 somehow?

Thanks!

There should not be able direct effect against the 2010 environment unless documented in the release notes or associated CU blog. 

Hello.
I had 3 projects for migration Exchange 2010 to Exchange 2013.
Issue you have to care when you coexisting is "AMBIGOUS NETWORK"

please read this link twice to make sure noting broke up.

http://blogs.technet.com/b/exchange/archive/2013/05/23/ambiguous-urls-and-their-effect-on-exchange-2010-to-exchange-2013-migrations.aspx

cheers!!