Technically what you assume is wrong, when an update is released it may contain both backend and frontend changes, another bad news for you is some backend changes do need mandatory/following frontend changes too... In theory you should NOT be editing the default template directly but have your OWN duplicate, which you can play on safely. You can check this forum for how to duplicate a theme properly and make your changes safe during updates.
https://docs.phpvms.net/installation/updating
https://docs.phpvms.net/customize/theming
`If you want to edit only select templates (also recommended), set the extends value to default. Then you only need to copy over the templates that you modify, instead of all them (this makes it easier to update phpVMS, and you also know what you've modified).`
But if you wish, you can try what you assume, it may work for one update and fail on the next. You will never be sure about that.
Good luck and God speed