Serveral times I had opened a project developed for SharePoint 2010 and I needed to make some changes for instance to make it work in Sharepoint 2013.

When you open it you will receive some problems regarding your dll references, what the hell ???

The solution for this is to make some changes in the xxx.proj file.

I was able to get my solution upgraded from a 2010 project to 2013 using the following.

Note that this will update your solution to use the new 2013 API.
First edit your .csproj file (for c#).

Modify the target framework to this:

<TargetFrameworkVersion>v4.5</TargetFrameworkVersion>

Add this a node for the office version, I put mine directly below the TargetFrameworkVersion tag

<TargetOfficeVersion>15.0</TargetOfficeVersion>

Update references
Reload the project and update your referenced assemblies.
If you haven’t specified a specific version they should already be referencing the v15 (SharePoint 2013) assemblies.

Do a find replace for 14.0.0.0 to 15.0.0.0. This updates any references on your pages, layouts, and master pages to the v15 assemblies.

Hope it helps !