There's a minor typo - if this is a direct copy / paste from your
module, it might be giving you problems:

Sub UpdateWkbk()
[SNIP]
alreadyupdate = True *****
For i = 1 To 10
sht.Range("J8:U40").Replace What:="=", Replacement:="=", LookAt:=xlPart, _
* * * * SearchOrder:=xlByRows, MatchCase:=False, SearchFormat:=False, _
* * * * ReplaceFormat:=False
* * * * alreadyupdate = True *****
* * * * Application.Volatile

[SNIP]

Private Sub ComboBox1_Change()
* * * *If alreadyupdated ******* Then
* * * Exit Sub
Else
* * UpdateWkbk
End If
* * alreadyupdate = True *******

Ed