This is something I’ve been meaning to do for a while, but it needs a bit of additional infrastructure to support properly. For now, I’ve made a project that uses the rounded version of the breve notehead, which I’ve attached here. You should be able to copy and paste your music from your existing project into this one (after setting up appropriate players and instruments, of course), and you should find that your breve noteheads then become rounded.
Alternatively, you could attempt open heart surgery on your existing project, though this you do at your own risk! If you change the file extension from breve-noteheads.dorico to breve-noteheads.zip, you can uncompress the Dorico project using whatever unzip utility you normally use, and in the unzipped folder you will find a file called scorelibrary.xml. You could make a copy of your current project, then do the same procedure to that, i.e. change the file extension to .zip, then unzip it. Replace the scorelibrary.xml file in your own unzipped project with the one from the project I have uploaded. To remake a Dorico project that Dorico can open, select the four files and folders inside the folder that was created when you unzipped your project, not the folder containing this files, and make them into a zip file again. Change the file extension from .zip back to .dorico, and now try opening that copy of your project. You should find that your project opens up with round breve noteheads. Note that if you do this, any changes you’ve made to things like font styles, master page definitions, etc. will be lost and you will have to make them again.
breve-noteheads.dorico.zip (60.7 KB)