Hello Daniel,
As said by you, extracting the data for 5yrs using repair full till DSO, as DSO is in overwrite mode there wont be any issue till here.
Now as you have added two new fields(appended fields) to DSO there will be change in record mode so you can just perform delta data load to cube and data for both new fileds will have data for past 5yrs as well.
Data which is changed will only be updated to cube and nothing will change.
Example: If you can change a sales order document which was already extracted to BW and which got changed after your enhancement then in the delta data load you will have data for both newly appended feilds as well which will be loaded through schedule delta data load to BW system.
If you have any doubts on this process please try in quality or development system before you proceed with this action in production system.
Hope this helps to resolve your issue.
Regards,
Venu Gopal