Here is some old VBA code for changing mleader numbers.
Sub ChangeMLeaderNumber()
Dim oAcadObject As AcadObject
Dim sOriginalNumber As String
Dim sChangedNumber As String
Dim oMLeader As AcadMLeader
Dim oAttDef As AcadAttribute
sOriginalNumber = ThisDrawing.Utility.GetString(1, "Enter existing MLeader Number to change: ")
sChangedNumber = ThisDrawing.Utility.GetString(1, "Enter new MLeader Number: ")
For Each oAcadObject In ThisDrawing.ModelSpace
If (TypeOf oAcadObject Is AcadMLeader) Then
Set oMLeader = oAcadObject
Dim sBlock As String
sBlock = oMLeader.ContentBlockName
Dim oEnt As AcadEntity
For Each oEnt In ThisDrawing.Blocks(sBlock)
If oEnt.ObjectName = "AcDbAttributeDefinition" Then
Set oAttDef = oEnt
If oAttDef.TagString = "CIRCLE" And oAttDef.TextString = sOriginalNumber Then
oMLeader.SetBlockAttributeValue oAttDef.ObjectID, sChangedNumber
End If
End If
Next
End If
Next
End Sub