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.

Reproduciton steps:

1) open new JCR Browser
2) create node and property within this node
3) trigger Edit property action

Expected result:
one can change property type

Actual result:
type field is read only

Developer notes:
'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).