I would like to chime in here - I have worked with Tom on this over the last week intensively - previously I had test flown the 737 in XP12 (after we fixed the major roadblocks of even getting if flyable, like making the flaps work, etc.) and I had tuned the flight model.
Tom worked hard on 3D modeling over the last months with little for me to do (I am too stupid for blender and those things) and now with him approaching the end of that work, I dove back into flight and system testing.
The more I test, the more stuff shows up that does not work anymore, because Laminar changed things - and these changes are all improvements in modeling systems, yet they still leave our "old" way of doing/overriding them unusable.
So in a very real way we have to redo a lot of the things we already did for XP11, it is not a quick conversion at all - and this is something we definitely underestimated when we initially said to "upgrade to XP12 for free".
The move from XP11 to XP12 is not an upgrade, it is in many areas a repeat of the dev work work we already did for XP11. And I am thankful for most of our customers to be very understanding of the fact that we charge a small price to compensate us for this!
I think the old 737-300 still has a lot of life in her, I thoroughly enjoy my "test flights" and find myself again and again to "just finish this approach" even though I was just testing some autopilot PID constant ... simply because it is fun to fly and looks great in XP12 .
Cheers, Jan