Hello and welcome to the forum.
The first question that immediately comes to mind is, why? The two Operating Systems are very similar with Ultimate having just a few more features.
Are you currently having an issue with your existing installation?
How did you go about getting to your present situation? Upgrade? Clean install?
If you have a Windows.OLD folder there is a method that Microsoft describes in this article here How to restore a Windows 7-based computer to a previous Windows installation by using the Windows.old folder
Perhaps, if you could describe your current issues, problems or concerns we might be better able to suggest a remedy or alternatives.