I have added a new text field which has an auto-calculation option set, but it only updates on records which have been altered since the auto-calc field was added.

How do I force it to update/recalculate the contents in this field only, so it will recalculate for records contents which were present before this new field was created and haven't yet been modified again?

On a calculation field, this is easy, but I can't see how to do it on a normal field with an auto-calculation option set.

Change the field into a calculation field, using the same formula (and the same type of result). Exit Define Database. Now change it back to auto-enter.

If you only want to update certain records, perform a find for those records, then use a Replace to update the field contents with a calculated result.