if you have software and databases of the different version, then you will run into errors, since the database structure is different, new tables/columns/rows may appear and they will be empty which can lead to the errors.
Why don't you just update your old website?
It sounds like any way you go your going to lose something, how much data was changed in those few days, alot or just a little. Right now your down i suppose, wouldnt it be better to just lose alittle data and get it backup to a few days ago and then move on from there ?
What kind of 500 error do you get?
500 internal server error or e500.php with the instructions to enable debug mode?
500 internal server error is a server related issue you will need to contact your hosting provider to resolve it.
Please follow this update instructions: https://developers.oxwall.com/forum/topic/24134 they still apply