I am 90% sure that the calls WOULD NOT transfer to department B. You can test it easily. Keep in mind the analytics portal updates every hour (bottom left corner says when it was last updated.)

To test it just assign a user to a newly created department, and check the past history for this new department and also the history of the old department.

Thank you Ryan Gosling! I ran the reports and the calls do transfer, which is a big problem. I need the traceability per department, and not just per user :(

Well, in this case I am sure it works as designed, and it will be hard to get it changed short term. It might be possible to find a workaround, but I need specifics to be able to think about this. Here is my best attempt to help you

Made-up example:
John Doe (with Ext. 301) needs to go from Department A to Department B.

Solution:
Leave John Doe (301) in Department A as a placeholder.
Create a new free User ext. John Doe2 (302) and add him to Department B.
Reassign the license from John Doe (301) to John Doe2(302). This is done form Phone System > Phones & Devices > User phones > Actions > Reassign to.

This should do the trick. Additionally I would maybe rename John Doe (301) to denote that this ext. is no longer used. Or maybe you could add a Job Title "Placeholder after Oct.8 2022)
I would also edit the permission - change his Role to custom role and remove all of the permissions to make it 100% clear that this ext. is no longer used.

The Bad thing about this approach:
John Doe will start anew - meaning he will lose any chat conversations he had in the desktop/mobile app. He will have no call history, if he had a custom voicemail greeting he would need to do it again for the new extension, etc.