How to Solution Prevent User Downgrade Sensitivity Label is changed

Technically speaking, it's not possible. Just like what Oliwer mentioned. What's your use case for this? Why do you want to prevent the label from being downgraded? If your use case is that you want to protect data inside of the labeled data, (ex. only the C-Suite can edit the data that is labelled "Confidential - CxO" then you could potentially create a label that is only visible to the C-suite group. Then add an encryption that gives the C-Suite full access while only allowing the rest of the organisation with VIEW only rights (no downloading, printing, copying ,etc). Since the rest of the organisation will not be able edit the document, they won't be able to change the label of the document.