Alright, we can try something like this:
 
Imports System
AddReference "System.Drawing.dll"
Imports System.Windows.Forms
Sub Main()
	iPro_Mat = "Material-iPro-Name"
	iPro_Des = "Description-iPro-Name"
	Dim Mat_List() As String = {"123456", "234567"}
	Dim Desc_List() As String = {"Desc1", "Desc2"}
	CreateForm("MyReplacingForm")
	AddLB(5, 5, "Material to replace:", 160)
	AddLB(5, 32, "Replacing material:", 160)
	AddCB(170, 5, Mat_List, 150)
	AddCB(170, 32, Mat_List, 150)
	AddLB(5, 75, "Description to replace:", 160)
	AddLB(5, 102, "Replacing description:", 160)
	AddCB(170, 75, Desc_List, 150)
	AddCB(170, 102, Desc_List, 150)
	Dim oBT As Button = AddBT(0, 0, "REPLACE")
	Dim PosX As Integer = MF.Width - (oBT.Width * 1.3) - 80
	Dim PosY As Integer = MF.Height - (oBT.Height * 3)
	oBT.Location = New Drawing.Point(PosX, PosY)
	AddHandler oBT.Click, AddressOf Me.Replace
	MF.ShowDialog()
End Sub
Private Sub Replace()
	Dim Val1 As String = CB(0).Text
	Dim Val2 As String = CB(1).Text
	Dim Val3 As String = CB(2).Text
	Dim Val4 As String = CB(3).Text
	If Val1 = vbNullString And Val3 = vbNullString Then Exit Sub
	Dim oDoc As Document = ThisApplication.ActiveDocument
	For Each iDoc As Document In oDoc.AllReferencedDocuments
		If Val1 <> vbNullString Then
			Dim ValX As String = iPropertyUser(iDoc, iPro_Mat).Expression
			If ValX = Val1 Then iPropertyUser(iDoc, iPro_Mat).Expression = Val2
		End If
		If Val3 <> vbNullString Then
			Dim ValX As String = iPropertyUser(iDoc, iPro_Des).Expression
			If ValX = Val3 Then iPropertyUser(iDoc, iPro_Des).Expression = Val4
		End If
	Next
End Sub
Private Function iPropertyUser(oDoc As Inventor.Document, oProp As String) As Inventor.Property
	Dim oPropsets As PropertySets = oDoc.PropertySets
	Dim oPropSet As PropertySet = oPropsets.Item("{D5CDD505-2E9C-101B-9397-08002B2CF9AE}")
	Try
		iPropertyUser = oPropSet.Item(oProp)
	Catch
		oPropSet.Add("", oProp)
		iPropertyUser = oPropSet.Item(oProp)
	End Try
	oPropsets = Nothing
	oPropSet = Nothing
End Function
Private iPro_Mat As String
Private iPro_Des As String
Private MF As Form
Private BT(0) As Button
Private CB(0) As ComboBox
Private LB(0) As Label
Private Sub CreateForm(Optional Name As String = vbNullString)
	MF = New Form
	MF.Text = Name
	MF.AutoScaleMode = AutoScaleMode.None
	MF.Size = New Drawing.Size(350, 210) 'Width, Heigth
	MF.MinimumSize = MF.Size
	MF.MaximumSize = MF.Size
	MF.Font = New Drawing.Font(MF.Font.FontFamily, 10)
	MF.MaximizeBox = False
	MF.MinimizeBox = False
	MF.ShowIcon = False
	MF.SizeGripStyle = SizeGripStyle.Hide
	MF.StartPosition = FormStartPosition.CenterScreen
	AddExitButton()
End Sub
Private Function AddLB(PosX As Integer, PosY As Integer, Optional Caption As String = vbNullString, Optional Width As Integer = 100) As Label
	Dim LC As Integer = LB.Length - 1
	If Not LB(LC) Is Nothing Then
		LC = LC + 1
		ReDim Preserve LB(LC)
	End If
	LB(LC) = New Label
	LB(LC).Name = "L" & LC
	LB(LC).Location = New Drawing.Point(PosX, PosY)
	LB(LC).Text = Caption
	LB(LC).Width = Width
	MF.Controls.Add(LB(LC))
	Return LB(LC)
End Function
Private Function AddCB(PosX As Integer, PosY As Integer, Values() As String, Optional Width As Integer = 100) As ComboBox
	Dim LC As Integer = CB.Length - 1
	If Not CB(LC) Is Nothing Then
		LC = LC + 1
		ReDim Preserve CB(LC)
	End If
	CB(LC) = New ComboBox
	CB(LC).Location = New Drawing.Point(PosX, PosY)
	CB(LC).Name = "CB" & LC
	CB(LC).Width = Width
	For Each Value As String In Values
		CB(LC).Items.Add(Value)
	Next
	MF.Controls.Add(CB(LC))
	Return CB(LC)
End Function
Private Function AddBT(PosX As Integer, PosY As Integer, Caption As String, Optional Width As Integer = 80) As Button
	Dim LC As Integer = BT.Length - 1
	If Not BT(LC) Is Nothing Then
		LC = LC + 1
		ReDim Preserve BT(LC)
	End If
	BT(LC) = New Button
	BT(LC).Name = "BT" & LC
	BT(LC).Location = New Drawing.Point(PosX, PosY)
	BT(LC).Text = Caption
	BT(LC).Width = Width
	MF.Controls.Add(BT(LC))
	Return BT(LC)
End Function
Private Sub AddExitButton()
	Dim oBT As Button = AddBT(0, 0, "OK")
	Dim PosX As Integer = MF.Width - (oBT.Width * 1.3)
	Dim PosY As Integer = MF.Height - (oBT.Height * 3)
	oBT.Location = New Drawing.Point(PosX, PosY)
	AddHandler oBT.Click, AddressOf Me.ExitButtonClick
End Sub
Private Sub ExitButtonClick()
	MF.Close()
End Sub
					
				
			
			
				
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Regards,
Mike
"Always code as if the guy who ends up maintaining your code will be a violent psychopath who knows where you live." - John F. Woods