Employee's number changed but other's phones still want to text old number

We had an employee go through a number change. Other staff have already updated their phones to either remove the old number or save it as a different contact (with a completely different name not using first name). When starting a new text, either individually or a group, iMessage defaults to the old number when entering the first name. In the case of individuals who had removed the contact completely, it still shows the name attached to the old phone number (from prior texts).

Ideally, we'd like to keep a history of the old text trails for reference in the future as this is a long-time employee who had a number change.

What can we do other than deleting all the old texts? This is confusing staff by not knowing which number is the correct one. HELP!