Hi Zhivko,
Please find our responses below,
1. So, I just need to install the new version, and no additional setting are needed?
Ans:- yes
2. How do I keep a backup of the old version though?
Ans:- just zip the authorizedotnet directory from your_moodle_folder/enrol/ directory
- If the new one doesn't work well - would I need to install the ZIP package of the old version again?
Ans:- no new installation is required as the version is same. Just you have to replace with old files.
- Would that delete the current settings for Transaction key, API, etc.?
Ans:- No