There's a migration guide[0]. Having done it this year, it's not too painful, depending on how much you've customized Bootstrap 3 to meet your needs. We've been migrating on a page by page basis, and so usually we wait to migrate a page until our designers come up with an update to the page.

[0]http://getbootstrap.com/docs/4.1/migration/

Smart way to do the migration!

If a codebase is already launched with bootstrap 3, what would be benefit of converting? (genuine question)

There's a few refinements in BS4. Refinements that become quickly inavouable. First and foremost the new flexbox directives all grid related stuff (mobile first). That's why I systematically migrate all my projects to BS4 even though it's not 100% needed.

Bootstrap 4 is built with Sass (Bootstrap 3's source code was built with Less). For my team, most of us had been using the Sass version of Bootstrap 3, and we decided to take Bootstrap 4 and create a customized version for our team. That task was made a lot easier with the source code being written in Sass.

In my experience, back when the switch from Bootstrap 2 to 3 happened, a big reason was being able to use sites like StackOverflow for answers.

Over time, all the answers became about Bootstrap 3 and it quickly became difficult to find help about Bootstrap 2.

There's a few transpilers out there but I found them not to work that well when I tried copy pasting some code off of bootsnipp. Specifically, carousels gave me issues.

As far as I was able to find, there doesn't seem to be a straight forward/automated process. Many things have been removed and do not have a 1-1 replacement. If

As far as I was able to find, there doesn't seem to be a straight forward/automated process. Many things have been removed and do not have a 1-1 replacement.