Oh, I see what you're saying. Took me a few minutes because I had no idea what those numbers were, but I googled them and I see you're talking about dealership OEM part numbers like this one.
https://www.toyotapartsdeal.com/oem/toyota~computer~engine~control~89666-20083.html
So, from what I can tell, it looks like the 20083-84 supersedes (replaces) the 20083. A lot of times Toyota will find defects in parts, and re-work them a little bit, and make that new part to fix that defect. At which point the old part number becomes obsolete so they make a new part number. I've never had one not fit when its from the dealership.
I have a 2000 Celica GT, and if I had a GTS, and I were looking for a new ECM (that's what Toyota calls the ECU) I'd feel comfortable purchasing that superseding part. I recently replaced my Sunroof, and had to buy a whole bunch of other dealership parts that were all superseded by newer part numbers to go along with my engine swap project and they all fit perfectly.
I've never had to replace my ECM so I can't say with absolute surety that it will be plug and play, but I'd be 95% confident that it should be. What I can say is that whenever I had any doubts about the parts, I just went down to my local Toyota dealership parts department and talked to the guys back there and asked all kinds of questions. At one point I even asked to talk to one of their mechanics who came right out and answered my questions. They were super nice about it. I bought a few bolts and gaskets (cheap stuff) so I felt better about getting free advice. You definitely can find cheaper prices from the online Toyota parts houses.
I hope that helps amigo.
