The file format of HyperCard 2.0’s stacks is different from that of stacks created with earlier versions of HyperCard. Stacks in the old format can be opened as READ-ONLY by HyperCard 2.0.
When such a stack is opened, the Compact Stack... option in the File menu becomes Convert Stack..., so that the stack can be converted to the new format.
ONCE A STACK IS CONVERTED, IT CANNOT BE RETURNED TO THE OLD FORMAT. Before converting a stack, you can (and should) save a copy of it by choosing Save a Copy... from the File menu.