Chosen Solution

My Macbook Pro 2014 13” housing is in tough shape and I would like it to look presentable again. I fear replacing it, however, since the serial number on a replacement housing will no longer reflect the original one. If I ever bring it to an Apple Store for a free repair program or a recall in the future, could they deny me service, or worse, carry me off to the Apple Store backroom to be interrogated by the Apple Police?

First Apple has a repair policy when the bottom cover is replaced. The servicer is required to note the original systems S/N on the inside of the lid in permanent marker. So I would recommend you do that so your system is following the SOP. As for Apple refusing service a replaced bottom cover would not be a good enough reason. Liquid damage or a mangled mess after a car has run over your system a few times. Of course obvious altering or mis-assembled parts is a red light so take your time and make sure you don’t damage anything and put the correct screws back in where they came from. Any of these actions would stop Apple as well as me ;-} Apple will do extended warranty service without question. Thats all you have now any ways.

You might consider a good hard case like this: https://www.ebay.com/itm/Quicksand-Metal

Yes. Apple will look for any and all excuses they can find to refuse service to you, and changing out the entire case will definitely be one of them. That being said, the 2014 MacBook Pro is also ~5 years old now, and any defects that would warrant a recall would almost certainly have cropped up already. If the housing is the only issue with it and you feel comfortable changing it out, I’d say just go for it regardless.