Engine/ECU compatibility
 

So I understand the compatibility issue with ECU/Immobilizer/keychip.
I also understand that certain years are compatible with certain engines (i.e.: a '99 cannot have an '03 motor put in it)
My question is: If the ECU/Imob/key is replaced with one from a certain year, can an engine from that year be used? Or am I overlooking something else?
i.e.: could I put a 2003 ECU and a 2003 engine in a '99 Boxster and have that work?
Additionally, I understand that the 2000 and 2001 models are backwards-compatible with the 2.5L engine, so could I put a 2001 ECU in a car from '99 or earlier?

My first thought on putting a 2000 or later ECU in a 1999 or earlier is that the ECU connections are very different.
2001:

http://986forum.com/forums/uploads02...1573252420.jpg

1999:

http://986forum.com/forums/uploads02...1573252453.jpg

Can it be re-wired? I'm sure it can, but it's not going to be a simple matter of looking at pics from Googles.

Next, Cable throttle vs Electronic throttle. -Adapt the throttle, or adapt the gas pedal.

That's the easy stuff for me to point out. I'm pretty sure that the '03 and '04 ECUs are going to be looking for some CAN Bus input, but I haven't looked at specifics. If you have a full car to pull parts from, it's probably not that bad.

Trust me it's worse than "that bad".

I have an 02' 2.7 running on my stock 2.5 DME

I believe there is a write up on Pelican that goes over engine swap compatibility vs model years. It may also go over ECU swaps exclusively but I have a strong feeling that will be part and parcel with engine swap compatibility

Yes I’ve read the Pelican engine swap write up, but I just wanted to make sure there weren’t any other compatibility issues I’m missing.
I’m interested in learning about this topic as a whole, but the specific application I have in mind:
I recently acquired a ‘97 with a fried immobilizer. Since I already have to replace the ECU/etc, I was thinking about putting in a newer ECU since I’m also considering an engine swap (3.4,3.6,3.8) and the ‘97-‘99 ECU only works with the 3.4.

Around every 2 years they've changed the complete ECU (DME, alarm…) setups. Also there are different software versions per setup. Mixing them up will get you in massive trouble. Also in 96 ROW to mid 97 ROW which might be also 97 US they used complete different memory chip technology. Not compatiible.

So, I may be missing something here, but it sounds like the only practical way of putting a non '97-'99 ECU into my car would be to change out all of the associated wiring harnesses as well?
I'm thinking I may just put a '97-'99 ECU in, since a 3.4 upgrade would probably be plenty sufficient anyways. Also, that would allow for very minimal work in the short term.