There is certain data about contact, companies, deals, tickets, etc that, once a value has been set, users do not want that value to be (easily or accidentally) changed.
This is already applied to certain HubSpot default read-only properties, such as First Page Seen, and, in a slightly different way, to Lifecycle stage which only be moved backward if it first cleared.
It would very useful if admins were able to apply either 'set-once' or 'clear to change' to custom properties, allowing them to enforce data integrity/quality.
An example use would be when companies want to enrich the source information around leads. Original source captures the channel that brought them into the CRM, but there is often additional information that can be associated with this, particularly for 'offiline sources' leads.
