Hi there,
The solution is simple, I have use it already with an older version of owncloud, as it was not working with PHP version 7.2.
You can install several versions of PHP and still use them with different webpages.
Just follow the links below and dig some more with google:
https://www.tecmint.com/install-different-php-versions-in-ubuntu/
that idea is to install the PHP version that works with your owncloud and PHP-FPM for each PHP version, then you should change the .htaccess file of your owncloud by adding :
SetHandler "proxy:unix:/var/run/php/php7.2-fpm.sock|fcgi://localhost/
in the section , as I remember.
This way you can still use your current owncloud version with the old PHP version, without having to upgrade your owncloud.
The owncloud upgrade method I find it complicated and not really smooth, as until now I didn’t manage to upgrade without re-upload.
Regards,
Alex