Any shift on HosPortal has two parts, and each has a history. The shift and each individual user assigned to the shift.
To see the history of the shift, bring up the shift dialog by clicking on it.
At the bottom you can see "show shift history" - this shows the history of the shift itself. On the right of each user you can see 3 small dots, the "context menu" for that assignment. Clicking on that will bring up a dropown with a number of options, including "show history".
In both cases, clicking on "show history" will reveal the changes made from creation of the shift or assignment: