What is the UX when I add additional builds to an existing external beta test in iTunesConnect

I currently have an app on iTunesConnect with a few hundred external beta testers using it. Important to note, we will have 2000 testers by the end of the month. I want to push new builds to this app- and this, I know how to do.

What I don't know is, what is the expected behavior for my beta users when I add a new build?

Our company cares a lot about user experience, and we don't want to have our current testers of our current build open the version that they've already installed, only to see it crash because I added a new build that i'm hoping will just update their current version automatically.

Apple does a good job of making a developer think this might happen. I've searched everywhere to find this answer in the docs- please help! After selecting a newly approved build to switch to in the External Testing portal, upon selecting Save, this alert appears:

(405 is the first build, 407 is the new build)

So, what happens when I save this- will users be notified that they need to update the app?

Will the "update" happen automatically for them if they've already installed the first build?

If they open the already installed version, will it simply crash?

If so, what can I do to prevent this from happening?

My team will likely want to send out 1-2 builds / week (of the same app, with fixes and improvements) to the same group of testers until we're ready to officially launch the app. I'd hate to think this would crash the app on them every time. As far as I know, there is no way for me to test this before performing this action- I'm already added as an Internal Tester, but that's a completely different UX in TestFlight (builds are made available to internal testers immediately after uploading)

If you think this has been asked already:

This is not a duplicate of this question- because I haven't attempted to send out the build yet. I want to make sure that linked issue does not happen to my testers!

My question is unlike this one where the user did not know how to properly increase his build number, unlike this one, referring to testing a new build of an app that already has a version in the app store, unlike this one which refers to a bug in the app store where a user couldn't initiate an external test after uploading a build, and unlike this one where the user just didn't know how the iTunesConnect portal works.