Hello
There is now an update-script available on [1] to update an existing installation of SGS 0.745 from PHP 5.3 to PHP 7.4.
Don't forget to make a backup of the entire installation directory. Things could go wrong. In this case simply restore the backup, analyse and start over again.
I tested on Ubuntu 16.04 with parallel installation of PHP 7.4 and PHP 5.3.29. In case you need to change OS in between the update, additional steps might be necessary. Report your experience.
1. Download the script and copy it in sgs/bin
2. login to SGS, clear the caches and run the script
3. download an archive and unpack it to sgs/simple_cache/update_to_php74
4. click continue to proceed with the installation.
5. at this point SGS stops working with PHP 5.3, so switch to PHP 7.4
6. click continue or start using SGS normally.
7. enjoy