Hi @Ross, Murray ,
You could try following steps to change a SharePoint web application from HTTP to HTTPS.
1.Add HTTPS Binding for the SharePoint Web Application
IIS Manager > select your web application > Bindings... > Adds...
2.Configure Alternate Access Mapping
Central Administration > Application Management > Configure alternate access mappings > Edit Public URLs > the HTTPs URL in any of the Zone like “Intranet”
For Reference:
How to Change a SharePoint Web Application from HTTP to HTTPS step by step
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