Since posting the original post, I found a few videos on installing a decoder in an Althearn RTR, so I can answer my own question, lol.
SoundTraxx: https://www.youtube.com/watch?v=MRT6URtdmJs
Model Railroading: https://www.youtube.com/watch?v=4HNfFfs21Mk
Solo Contracting: https://www.youtube.com/watch?v=CYOrmW9F1U4
K4JW: https://www.youtube.com/watch?v=dfRnHFneI_E
If the manufaturer says remove the couplers, ya gotta remove the couplers
This diagram from HoSeeker shows the coupler springs to be part of the shank, so unless you lose the coupler, you should be ok. When I am disassembling something with small parts, I put them in a container as I work. That can be a plastic disposable cup, small bowl, old ash tray, etc. Unless you knock it over they should be safe
https://hoseeker.net/assemblyexplosionAthearn/athearnrtrrs32005.jpg
Specifically, it is an Athearn 94054 Erie Lackawanna 929. Although my railroad is largely Amtrak and Conrail set loosely in the 1970s through the 1990s, there was an Erie Lackawanna line running through the town I grew up in, and when I saw this locomotive, and the box said DCC QUICK-PLUG EQUIPPED in big bold letters, I nabbed it for sentimental reasons. Now, I need to remove the body shell to install the decoder. Therein lies the problem. The single sheet instructions are rather vague in describing how to remove the body shell, and I can't figure out how it is removed. It says to remove the Kadee coupler boxes, but I know if I do that, microscopic springs and parts are going to be scattered around the world never to be seen again. I want to know clearly what I'll be doing before I start. Can anyone point me to a clear and thorough and detailed description on how to remove the shell of this locomotive? Thanks much!