Isn't constructing a new language the ultimate breaking change :-)
No not really, if your language is intentionally designed to be a foundation for other languages. So many lisps are trivially host languages for other languages at their core. Even JS is at this point, with the widespread use of Babel and various bundlers. Those languages themselves are seldom very different underneath that.
There will be two similar yet different languages in use until everyone migrates to the new version. This process could happen quickly or not at all.
Now, perhaps the problem in your eyes is the old version isn't being maintained. But I see the opposite: maintaining two versions indefinitely is the problem. The Haskell community is not infinitely big, nor growing fast enough, that if e.g. 1/2 went to Idris and 1/2 stayed, it wouldn't be catastrophic.
Making the changes we need to make, and then making it as easy as possible for everyone to migrate, I think is the best way.