Bumped into this problem where I was trying to convert old codes from SharePoint 2010 to SharePoint 2013.

One issue that you will encounter is that all your Element Files being uploaded to 14 Hive folder (the old 2010 path) rather than the new 15 Hive folder.

If you deploy your solution directly into a SP2013 environment, you will encounter reference link broken or file not found due to codes/files not being deployed to 15 Hive.

One Workaround is to actually deploy your solution with switch -CompatibilityLevel All (for detail please refer here ). However, you can’t always remember to do this steps and in order for you to really upgrade your solution to SP2013, You need to make some changes to the manifest file. And steps below show you how to change it permanently and only stick to 15 hive folder.

Resolution

1. Open your SharePoint solution via Visual Studio.
2. Expand the Package folder and Double click “Package.package“
3. Under Properties Panel, change the property “SharePoint Product Version” from 14.0 to 15.0
   
4. Package it and done!

P/S What I additional did to make sure there is no reference link broken is to Find ALL “_layouts/“, “_CONTROLTEMPLATES/” and replace with “_layouts/15/” and “_CONTROLTEMPLATES/15/“