Seeking Insight about how adding new PHP releases via EasyApache

My server has PHP 5.4 through 7.2 installed. I need to add 7.4, 8.0, and 8.1. I see where they can be enabled in EasyApache. Since I need to keep 5.4 around for another month or two for a legacy App, I just want to confirm that adding the new versions is not going to impact the currently installed versions? Also, when new versions of PHP are added, what happens to existing domains? Do they continue to run the version of PHP they were using before the new additions or are they automatically updated to the latest versions, i.e. 7.2-->7.4? TIA!