Give the replacement model the same name as the one you want to replace and put all it's files in the correct folders to get it to work.
TF2 checks overwritten models on a bsp by bsp basis. As long as the custom prop is packed into the bsp itself with something like compile pal, then it will replace the model when the map loads and revert to the stock model upon map switch.
Something to keep in mind though is that some players with some settings don't see replaced models using this method. I've talked to multiple people about this and done a bunch of testing but have no idea what the setting(s) are that causes this to happen. If you can figure it out that would be amazing.