Good GPS receivers allowed changes to datums. Furthermore, there is a transformation possible between ellipsoids if you have their exact parameters.
Source: I worked at Trimble.
Source: I worked at Trimble.
Edit, read the rest of the article to put the linked section into context. The commies are trying to obfuscate a map grid? That's just nutty. Are they really going to arrest everyone who is found in possession of a magnetised needle?