old version cdt overwriting new data in the table

I have updated a cdt and added an extra column to it. There are few thousands of instances in the old version of the cdt where the extra field is not present. In the process instance as well the cdt is showing in cap sign which means it's referring to the old version. I have done a data migration to populate that extra field for the old instances. However, while writing the data to the table through the process, even though the new column is not present in the old cdt, the database column is getting overwritten by null value.

Is there any way how to avoid this scenario?

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