Well if your actually going to go through with it, make sure you check out omzy's
script for automating most of the work. You'll have to edit it up a bit to fit your method but its not so bad.
Also he has a pack that he did a few years back with the backgrounds already completed.
More recently I have actually finished my own upscale project as well using my own method that I found to be superior in quality and actually touched up a lot of the scenes by hand to make it more of a seamless experience. You can check out my work
here.
So if your going to redo your this project in your own style I'll help as much as I can with pointers and things that I have found to make things easier. However your going to want to test all of your scenes in game before deciding on a process because what looks good in photoshop might look tacky or unfit in game. In particular your scenes might be a bit too blurry and noisy and will contrast poorly with the 3d models which are sharp and devoid of any shading or noise. Blurring sounds like a great idea at first, I used to think so anyway but in execution it always panned out poorly and made the upscales feel too obvious. Trust me save yourself days of work and test first before you get too excited.
Anyway that's just my opinion on it. Good luck.