I think I have given you enough links. Now copy your file elsewhere and give a new name. Now open the file and go ahead ignoring the message. The file will open alright. Only thing is that you will not be able to edit the mechanical symbols which have been done under annotative scaling. Probably you have to erase it and re do it if you wanted to change it.
Hope my 2 cents of advise helps. Those links will definitely help you better.