How to copy data from Process Model properties variable tab?

Objective: copy data from Process model properties and store the old data copy into same table(Multiple of updates done using interface) before new data update.

I have "process start form" tab with - Update_Ticket_Interface

This interface to update ticket on selecting table row Interface shows details to update ticket.

Here, want to copy available data into a variable before form submit/update. 

How to copy data from Process Model properties variable tab?

Here, i am able to update only one record, not sure how to use process model to get copy of old data to store into same DB.

i have process model as, Inputtask-startNode --> Write To Data Store Node -->End node

In below screenshot, i There are two variables, PTS_TicketTest(For record Update-working) & PrevData(ForRecordOldDataCopy-GettingNull)

PrevData tried to store in another Write To Data Store but getting null data, as PrevData didn't get data.

Please let me know if any clarity needed.