You can upgrade to the beta build at any time, but the library sync will need to restart. Currently synchronized assets will be retained and that work won’t be re-done.
I personally run the beta build for our Official Family Library (which would cause Consternation from Elders and Nieces if it broke, so that should give you an idea of how stable it is). I strongly suspect you’ll see fewer issues with v1.0.0-beta than v0.9.1: I’ve fixed a ton of bugs and added a ton of features since last November.