Hi, I would like to know if it's possible to upgrade from LimeSurvey version 1.91 build 11026 to the latest stable release without deleting our current surveys.

Hmmm, I don't think anyone can answer this. I don't think that anyone tested the update process from a version that is over 9 years old.

What I would do: Create a copy of the database you are currently using. Install the latest version of LS 3.x as a parallel installation (not over your current installation and use the copy of your database for this installation. See what happens...

What will not work: Your templates/themes from 1.91 and the work arounds used. This is for sure. Now the rest is just trying.

But if you follow my advice you won't destroy anything. If the update fails, you still have your old instalation untouched and can go from there.

Maybe you will have to go through other major releases, e.g. 1.9x to 2.06LTS to 3.x for example.

If you don't have many surveys and don't need the history of the old results, I personally would always go for a fresh install. But of course, if you have many surveys that you need in the new installation and/or many users, it might be worth a try to test the update process in a parallel installation and see what happens.