To successfully migrate from Dynamics CRM 2011 to 2013, ensure you know how to convert a managed solution to unmanaged and utilize a legacy check tool before proceeding with the migration.
You might need to modify solutions during migration. But if that solution is managed, you cannot edit directly. If you have the unmanaged version in development environment, you can edit it and override the production managed solution.
Before upgrading from CRM 2011 to CRM 2013, need to review migration doesn't contain any legacy code.