If all you want is to upgrade syslog-ng, then run the new version with an unchanged config file (on a separate test system even), and observe "configuration" change warnings during startup.
The one you'll definitely get is about the '@version' line at the top of your config. This probably says '@version: 3.19'.
Don't fix the version declaration until you fix the rest of the warnings: syslog-ng operates in compatibility mode with the old version, and gives you instructions about what was changed since the old version. These warnings are emitted during startup, so look for log messages generated by syslog-ng right after starting up.
For instance, if a default changes between your old version and current versiony syslog-ng would tell you something like
WARNING: the default for whatever option was changed in syslog-ng 3.25, please adapt your configuration
What this means is that you might need to be explicit about the setting in your config and not rely on the default. So add the option and its desired value to the config explicitly. Or if you thing the new default is ok, just ignore the message and fix the rest too.
These days the number of config changes is pretty rare, so you might not have any warnings, except for @version.
Once you handled all the warnings just update @version and you are done.
Bazsi