Read original post by Jani Nevalainen at .NET Junkyard
Thought I give a short summary for you who are interested in bringing your WP7.5 app to WP8 for what you should look out for.
In general you shouldn’t recompile your WP7.5 app to WP8, for two reasons. Firstly, WP7.5 apps will work on WP8 as is, without any need to rework the code. Secondly, recompiling apps is only going to get you in trouble, because there are some changes in the way .NET Framework and CLR has been implemented on WP8 compared to WP7.5.