Need to roll back to 2013.3

Hi

I recently upgraded to 2015.5 without a subscription and now my file is read only because I am unable to update a serial number, so I spoke to MYOB and they were going to send me a file of 2013.3 but I've not heard from them.  I still have the original 2013.3 file can I just reinstall over 2015.4 and it will work?

Thanks, regards

Elle