https://social.technet.microsoft.com...eneralprevious

sees the orignal post here.

This question has been asked many times as i have seen and one of the best answer for this question is
"SharePoint 2010 object model contains many changes and enhancements, but our custom code will still compile and, will run as expected. You should however, rewrite and recompile any code that refers to files and resources in "12 hive".

Original Answered here: How would you re-deploy the old custom solutions in SharePoint 2010.What Changes are needed to the old Solution files. - .NET, C#, VB.NET, Sql Server

for more detail read this article: Backward Compatibility of SharePoint 2010 Object Model on migration