We have a List with 4 columns. When we need to add new column without loosing data, we have to change a property in the LISTINSTANCE.
after adding new column, we have to get PROPERTIES in the LISTINSTANCE.
Then we have to change the "Deployment Conflict Resolution" as "NONE"
After that we can deploy the solution.
It will add new COLUMN to the LIST without data deletion.
Also, we add new COLUMN as REQUIRED, but for OLD items it was not an issue.
But for a New ITEM it is mandatory.