We are upgrading SharePoint 2010 projects to 2013. Currently working on master page project. This project has an module with the masterpage file, code in this file is copy paste of v4.master with probably some minor additions/changes. Suppose if we upgrade this project to SP2013 and deploying this masterpage wsp, the entire site gets messed up.
Is there any solution to this? Or is it that we have to go ahead with using seatle.master only?