There is only readonly field when editing properties in new JCR Browser app (jcr-browser-app). But it was possible to change the type in old one, and it is actually possible to change the type in Configuration app.
1) open new JCR Browser
2) create node and property within this node
3) trigger Edit property action
one can change property type
type field is read only
'jcrType' property is set to 'readOnly' in ui/jcr-browser-app/src/main/resources/jcr-browser/dialogs/editProperty.yaml
Note that in old JCR Browser (jcr-browser) and/or Configuration app this is possible, but when trying this please assure that you erase the value field while changing type (value could be changed in next round).