diagnosis code added to already paid claims

[reliable@epix.net]

I do billing for a Physical Therapy practice.  I have a patient that is mid treatment and therapist added an additional diagnosis code.  When I inquired if addition was from today forward
they stated no, it should have been on all dates of service.  Previous dates of service have already been paid.  Do I need to resend these as corrected claims?  Informational purposes only?

[Michele]

What type of insurance is it?  I'm asking because if it is a workers' comp or auto accident that additional diagnosis could make a difference in the case.  If it is just an additional diagnosis and not the primary diagnosis, it may not be necessary to bother to do a corrected claim.  If it is a commercial plan and they've already paid the claims, you could contact them and just ask them if they want the correction.

[reliable@epix.net]

They are commercial claims.  Thank you for your help.