I would typically do this with the "hiding" feature. Still put two fields on the layout, but it's the same field just with different settings.
For the "hiding" feature, I'm referring to the "hide when" section of the Data tab on the inspector.
Hope that helps,
In addition to Chad's suggestion you could "toggle" a tab control, slide control, or a layout with different control settings.
You could also leverage the OnObjectEnter script trigger to Go to a different object or field based on the contents of a variable.
To prevent editing i would leverage the security/permissions settings at the field level as obfuscation is not effective security.
don't forget to also evaluate Get(WindowMode)