With version 12, Kentico began offering MVC as a first-class solution, in addition to the Portal Engine approach. If your site is built on Kentico 12 MVC, we have great news: an upgrade to Kentico Xperience 13 will not require a complete rewrite.
Versions 12 MVC and 13 MVC use much of the same core functionality. Each site should be evaluated on a case-by-case basis—particularly your integrations. But the overall upgrade path should be relatively simple and will result in significant performance gains.