Samsung now lets you remap the Bixby key on older Galaxy phones
Promised and delivered. Samsung is rolling out an update for Bixby that lets you remap the button summoning the voice assistant. The update comes in the form of a Bixby app update so don't expect a full-fledged firmware OTA upgrade.
The cool thing is that you can assign not only a certain pre-installed or a third-party app but also a command sequence. However, Bixby should be assigned at either single or double press - whatever suits you better.
You can do that by going to in the Settings and then Advanced features > Bixby key. Also, keep in mind that a long press will always launch Bixby Voice.
Speaking of which, the Chinese version of the Galaxy S10-series has the option of swapping the "Hi Bixby" hotword for a one to your choosing. It's still unconfirmed whether the option will be made available for the global market, though.