Splicing together new and old columns

I am modifying a legacy board that already has tons of rows.

I cannot modify existing columns (i.e., change their type) because rows already exist that make use of those columns. Therefore I am forced to hide create new columns to represent existing attributes.
This means that I now have two columns for the same attribute (e.g., Requester). Legacy column “A” was a drop down; new column “B” is a short text).

How do I force a “merge” of the two columns into one (i.e., for new items show info from B and for existing items show info from A) ? It’s confusing for users to see two different columns in a single table representing the same information – but with tons of blanks.

If this is not possible, how do I efficiently create a brand new table using the new format and populate the legacy information into it?

Thanks,
Eileen