You can open stacks made with earlier versions, however, a padlock appears in the menu bar indicating the stack is locked. You will be able
to read it.
To be able to make changes in the stack it must be converted to the newer HyperCard format. You should always make a copy of the stack before you convert, because you can't convert it back and you won't be able to use it with an earlier version of HyperCard.
Once you have made a copy of the stack, use the Convert Stack command from the File menu. Convert Stack takes the place of the Compact Stack command when you are working with an unconverted stack.
Click OK at the conformation dialog to convert the stack. The padlock will disapper from the menu bar when the stack has been converted.