Oh ok, that's quite unfortunate. I think it's a little strange, Plesk should consider adding this feature. It makes keeping up to date with default php versions difficult.
For example, lets say you have PHP 5.6 as the latest version on your server. Then PHP 7 comes out and becomes stable enough, you would want PHP 7 as the default PHP on new accounts. It would be bad business practice to change and sync everyone else that was already on PHP 5.6.