Upgrade from Snow Leopard to Mountain Lion in non personal & large scaled development environment
Tipps what to consider when doing an upgrade:



Before you start


Take your time for a good backup! - You can regard data which is only
located on one harddrive as non existend.
Start with one system first and try out everything before doing the
major upgrade.




Installation


Consider it takes way more time than it does at home! The
MountainLion Download took >5 hours because of a symmetrical internet
connection. Installation time was ~1 hour (MacMini 2009 - 4Gigs Ram
1067). + xCode Downloadtime ~1hour
Before you install the new xCode make sure all your projects and your old xCode Dev Folder are
backupped.
When the installation routine asks you to delete your old xCode do so
(I didn't do it and now I have to manually assign each project to the
new xCode. I've to delete the old version manually too and it's a bit
more complicated if you have to do the research which files you are
allowed to delete and which not)
Then you have to download the different xCode
libraries. There goes at least 1 additional hour.





Time of the downloading- and installation process so far: ~8 hours. (Keep in mind we've a slow internet connection)




Projects


Copy your projects and don't run your conversion and tests on the
original.
When you open the project you'll probably receive lots of warnings.
Don't Panic!
When clicking on the "Validate Project Settings" warning xCode will suggest to convert your
project for you. I've done this for 3 iOs applications (Build Target:
4.3) now and it worked every single time. Let it create a snapshot for you. Just in case.
If your app has already been code signed you'll probably receive the
"Target Integrity" warning. In every single case it worked for me to
edit the path for the "entitlement.plist". (Just click the warning
and edit the path at "Code Signing Entitlements")

It may happen that you've to change the deployment-target. To do so
go to the Project Navigator (cmd + 1), click on your
project's name and then Summary. There you can edit your
Deployment Target.
If you get warnings on deprecated methods you should search
stackoverflow.com for your particular case. If it has to do with
NSURLConnection look here.


To be continued...