We have a multi site project in Sitecore 6.5 Web Forms which requires decent refactoring, architecture redesign and we want to upgrade it to sitecore 8. Of course it will be easier to start something brand new, but the project is huge and probably it won't be possible to rewrite it to MVC in completely new solution because of time that will be required to have all sites up and running. That's why we started to think if it is possible to keep the old solution and add there MVC project which allows us to write new sites in MVC and slowly refactor the legacy code in the meantime.

I saw John West's post about using Web Forms and MVC in a single solution <http://www.sitecore.net/learn/blogs/technical-blogs/john-west-sitecore-blog/posts/2012/06/using-web-forms-and-mvc-in-a-single-solution-with-the-sitecore-aspnet-cms.aspx> but I need to be certain that it will not cause any problems in the long run, before we propose this solution to the customer.

Does anyone have experience with keeping both type of projects in a production application? Is it possible to do a migration sublayout by sublayout without any risk? Can we have in one time part of the site in web forms and part in MVC (of course with pages separation to only Web Forms or only MVC controls)? Do you know about any constraints that we should be aware of?