Any compatible smartphone running Android 6.0 (Marshmallow) or Android 7.0 (Nougat) now has the ability to make use of the Google Assistant, following Google's confirmation of the rolling out of Google Assistant support to more devices. As to be expected, this will replace Google Now on Tap with the Google Assistant. However, if you would prefer to revert back to Google Now on Tap, then it seems there is a way you can, thanks to a thread on Reddit which spotted the method on how to do it. Curiously, the method also seems to work with the Google Pixel and the Pixel XL in the US. So if you own either of the Pixels (which come with the Google Assistant activated at startup), then you can also change the settings to start using Google Now on Tap instead. Here's how to do it.
To change from the Google Assistant to Google Now on Tap, head into the smartphone settings menu. At which point you will need to head down and click on "Languages & input." Once here, click on "Language" and then on "Add a language." Of course, what you click on from this point onwards is totally up to you as it will alter the displayed language on your smartphone. So click on whatever language you are able to read/use to ensure that the smartphone remains in a usable state. If you are an English-speaking user, then you can simply click on "English" and you will be presented with a list of English-relevant regions. The key is to select one of the regions which the Google Assistant has not widely rolled out in yet. In the images below, you will see the "British Virgin Islands" as an example of one of the workable regions. Once the new language is added, you will then need to remove the old (current) language by heading back to the "Language" submenu, hitting the three button options icon (top left), clicking on "Remove", selecting the original language option, and hitting the trash can. At which point you will receive a warning that you are about to change the language.
After which, the Google Assistant will have been replaced with Google Now on Tap. Of course, you can undo all of this by going through the same motions above and reinstalling the original language once again.