Hi
when you ask "do I need to run upgrade once I have changed the version" do you mean run the xerte upgrade after changing the php version on the server or something else? If you do mean do you need to run xerte upgrade then no. You only need to run the xerte upgrade after upgrading the xerte code/version already installed.
If you have upgraded PHP on the server where you had a running Xerte install and now seeing a prompt to run setup then it's probably because you have some missing PHP components and therefore the new php version isn't connecting to the database or because something else has changed in terms of permissions etc. Check the PHP requirements via our release notes
xot.xerte.org.uk/play.php?template_id=81#systemrequirements
HTH
Ron