Question on moving from kits to bundles

I am working on a project that will possibly be integrating a product kit for our 2013 CRM on premise instance.

At years end, our organization  will be upgrading to CRM 2015.  I understand that the Product Kits are replaced with Bundles so I was wondering if Kits easily migrate to bundles or will I be required to rebuild any Kits that were done in 2013 as new bundles ?  

We are also looking at adding some  entity relationships to the product kit but was not sure the level of effort for migrating to 2015 bundles ?

Should we consider custom entities instead of implementing the product kit since we will be shortly going to 2015 or will it be rather easy to upgrade kits to bundles ?

Thank you,