C# regex replace a name group with another value

I have a document of text that I use a regex to look for a specific line.  Within this specific line, there are named groups.  For one of these groups, I need to replace the value with one that is supplied by the user.  I cannot do a simple find/replace because the replacement is only applicable in the line of text matched by the regex.  I am working with EDI files.  The example I'm using below is simply an example, not a properly formatted EDI document.  The "edi" variable is my source document.

const string lastNamePattern = @"(?<NM1>NM1\*IL\*1\*(?<LastName>[a-zA-Z0-9]{0,60})\*[a-zA-Z0-9]{0,35}\*\*\*\*MI\*[a-zA-Z0-9]{2,80}~)";

const string edi = "ISA*PERRY~NM1*IL*1*PERRY*CHRISTOPHER****MI*1234567890A~IEA~";

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I want to replace the named group "LastName" within the "NM1" group with another value.  I need to return the whole edi string with the replaced value.  Any help would be appreciated!