If you delete a parameter from an action, anything calling that action will still show the value of the deleted parameter, but it will be disabled because it's now obsolete.
Currently, if that parameter has multiple lines of text (which is common when passing things like Javascript), it is not possible to copy the value (although you can for single lines of text). So you're left either having to rewrite the text, open a previous version of the module and copy from that, or undo the action update. This can be a real pain if that action is called many times.