Not field size... I'm talking about copying an entire column to another column.
If this were a one-time cleanup process, to correct something, I'd say it's worth pursuing (perhaps). However, if you intend to do this as a regular part of a workflow, NOW is the time to stop and think carefully about the wisdom of such an approach. Frankly, duplicating columns is almost never a good plan. I'd say "never", but well, one should never say never.
Ah, heck, it's never a good approach as part of an on-going process.
Copy/paste is a MANUAL process, and as such relies on the care and precision of the user to accurately select, properly copy and accurately paste the data.
Append queries are the proper tool for a relational database application in any event.