Great question, I’ll include some info in the beta post but generally most of the changes are to JavaScript and the use of Jquery. But there are some class changes and html structure changes too.
Blocs 4.2 now uses vanilla JS (no jquery dependency) for the blocs.js and scrollFX.js, this means the same JS files Blocs generates with every site can be used by bootstrap 4 and 5.
If you are building with bootstrap 5, Blocs will scan all of the additional JS attachments and check for Jquery references and then add Jquery to the page if required.
So if the custom Brics you use need jquery and you build with bootstrap 5, you are going to lose the main benefit. The transition for third party Brics may take a little longer and some may always require Jquery.
Most of the classes that have changed are utility classes, and the navigation now has an additional container in the structure.
Blocs will migrate, projects, page templates and Blocs to BS5. The only thing it doesn’t do yet is migrate custom brics (html), that are added to a page, as this may break the functionality.
I’ll work with the devs to find the right solution to this once beta testing starts.
Finally Bootstrap 5 is still a beta so my advice is to play around, don’t move anything important to Bootstrap 5 until the dust settles 
