Now I have a clearer picture of your operation. It seems your company does not want to incur the cost of board redesign, and I thought it was the initial investment of the capital equipment that had them scared. You stated, "Unfortunately I can't get to the supporting defect data that I was used to seeing on a daily basis." You say you don't have failure data? Why not? What sort of data collection are you doing? I don't know that you can make a sound decision that going to SMT will fix your problems. It probably will, but your issue is masked right now because of your lack of data. You have to collect the data first to find out what the underlying problem with these boards are.
You need to find out why the boards are failing. Is it the parts, the sockets, or some other issue? It may be that it has nothing to do with the older PTH parts and you could save your company a lot of money by fixing the issue with the PTH boards. If it is the parts, that should be argument enough that conversion is warranted. However, if its not, conversion, at this point may not be. How is your parts sourcing for these PTH components? Is there plenty of supply?