Smriti,
As i understood update flag is also field of your table . That would be better if you select the data from Ztable and validate with new data (Uploaded by Excel in internal table) via table keys, and then update new and old both flags in internal table with new data.
After validation and populate entries in internal table you can use Modify statement . If data with exist with Old Flag it will get modify in table else new record would be created.
Deependra