It looks to me as if you've done all the heavy lifting.
This code loops through the tasks, saves the current LateFinish, changes the ConstraintType to ALAP, saves the revised LateFinish, changes the ConstraintType back to the previous setting and moves to the next task.
Sub pdqbach()
Dim tsk As Task
Dim varConstraint As Variant
For Each tsk In ActiveProject.Tasks
If Not tsk Is Nothing Then
If Not tsk.Summary Then
tsk.Date5 = tsk.LateFinish
varConstraint = tsk.ConstraintType
tsk.ConstraintType = pjALAP
tsk.Date6 = tsk.LateFinish
tsk.ConstraintType = varConstraint
End If
End If
Next
end sub
Not sure what else you need done. Please advise.