There's a large amount of effort going into backwards compatibility.
Except for some really well motivated very advanced language features (eg view bounds) most code should be source compatible, and Scala 3/Dotty is able to use Scala 2 libraries and there's plans for forwards compatibility with Scala 2.14 (2.14 being able to use Scala 3 dependencies)
The big exception is macro's, because they depend heavily on Scala 2 compiler internals. A lot of things you require macro's in Scala 2 will become language features in 3, but for other cases the macro system for 3 is still in active development.