If you've changed anything, the upgrade would lose those changes if you've made any changes to any of the core files...
You need to find and compare all the files in the new release which have changed so if you're familiar with unix and scripting, you can run commands to parse the new files with what you have now. Going from 1.1.9 to 1.2.5 (IMO) would be a lot of work if you have a lot of changes assuming you'd want to keep them all. You also have to factor in any extensions you have not supported in 1.2.x.
Another option is to just load the newest version (1.2.5) and migrate your data over which would work. You'd have to weigh which is easier and how much knowledge you have with the O/S, Abantecart, etc...
As I've mentioned in other posts, I'd suggest setting up a test environment first, then try to upgrade a backup of your existing site first in that test environment....then move forward with your production after backups...