Modifying user description

I am having trouble finding a script for modifying the description field for user accounts in Active Directory.

For example the description field has:

Oldname, 1234567

and it needs to be changed to:

Newname, 1234567

Keeping the last part of the description field.  The last part of the description field "1234567" will vary base on user, but should for the most part, have 7 characters at the end.

I have researched trimStart(), but am not quite sure how I would implement that for a script.  

Any help would be appreciated!