It is likely, but we can't absolutely guarantee it. We don't want to - because we don't like breaking projects - but sometimes it is necessary. Being in-development beta software, we want to fix problems early rather than end up stuck with problems forever, when even more people might be using it.
However, generally they're very minor things that are easy to fix. For example from the last build:
typo bug caused the "Is between angles" condition to use the wrong ID. This has been fixed but any existing projects using the condition won't open until you change the ID in the XML from 13 to -13.
That can be fixed in notepad. It's likely other incompatible changes we make can be fixed in notepad as well, because all the files are XML format. So anything we break should be fixable.
Another example is when we introduced animations the Sprite object became incompatible - but I added code to convert it automatically - so noone noticed :)
So hopefully at most there will just be minor inconveniences along the way.