This is possible theoretically to change the URL from the backend even after the delivery is sent. There is a file on the app server which stores the mapping of urls with deliveries. The trick is to identify that file and correct the URL there.
1>. Convert the delivery primary key to hexadecimal
2>. Go to the folder \var\<instance_name>\redir\url\2018 and search for the hexadecimal value in step1.
3>. There will be an XML file like 215dc4.xml (example only) , please open that file and change the URL you need to change and save the file.
4>. Restart Web and Apache services and when someone now clicks on your email Link , it should go to the modified URL now