I have tried to use the Change Version (ArcGIS Pro 2.4 API Reference Guide) method on my current map and do not get the results I would expect. I copied the code example directly into a button action. The UI doesn't show that the version has changed and there are no errors reported. My Map has layers using the classic versioning model. Is there something else that needs to be done to the UI to refresh? Is there another way to get all the layers in a map to point to a different version?
If you haven't done so already, I would take a look at toVersion.GetName() prior to calling Map.ChangeVersion() to verify that the value is expected.
Outside of that, I don't have any other ideas. I set up an Oracle 12 environment that used classic versioning, and ran the sample code to switch the version from SDE.DEFAULT to the next version in the list (GDB.foo). It all works correctly, and the Map refreshes afterwards.
You may need to open an incident with tech support to figure out why this isn't working for you.
--Rich