We have active shop which is created on 2.3 (2.3.0.2.2) opencart version. Now we have created new show design using Journal3 on 3.0.3.2 opencart version.
And now we are having problem importing everything from active shop to new. When using default Backup & Restore tool I am usually having errors like
or something like thisFile could not be found!

But when I am using Export/Import Tool it wont work anyways and usually getting errors like
or that some categories for products arent defined or something.Invalid header names
I would like to know is there some more proper ways to Export and Import data from older version to newer?
We don't upgrade old version because there we have installed extensions whom's works only with 2.x versions ; )