Free app in Google Play Store allows Bixby button to be remapped to Google Assistant
It is a simple, basic app that developer Dave Bennett calls a test. He says that he might expand the capabilities of the app if there is any interest from users. After installing the app, you will have to allow it to access your usage data. A simple toggle switch on will do the trick. One thing we should point out is that after installing BixReMap, when activating the button, you will first see Bixby start up before the app takes over and runs Google Assistant on top.
Bennet has been open to suggestions from those who have installed the app. If you install BixRemap and have some ideas on how to improve it, make sure that you pass them along to him.
Download BixRemap by clicking on this link.
source: Reddit via AndroidAuthority