As was pointed out here clearing the SyntaxInformation isn't trivial to do.
The best I was able to come up with for clearing it was:
SyntaxInformation[f] = { "ArgumentsPattern" -> {___}, "LocalVariables" -> {"Table", 0}, "ColorEqualSigns"->{0, 0} }; SyntaxInformation[f] =. But a) this shouldn't require two calls and b) this doesn't handle the "OptionNames" parameter. I have no idea how to clear that.
How can we fully reset the SyntaxInformation?