Since SSI Web 8.4.8 until the current Lighthouse Studio 9.5 release we have written code that will convert an existing survey database to the latest version. That said I'd always recommend some caution when updating a live survey, especially to a major release. If you need to update a live survey I'd suggest doing it at a low traffic time or temporarily closing the survey until you can update and test it.
To be extra careful you could upload your existing survey to a test location, collect some test data, update it and then test.
There can be some big changes when we make major updates that might make updating a live survey more tricky. For example CBC had a major visual update when moving from 9.3 to 9.4. When the last number changes (say 9.5.0 to 9.5.2) that usually just means bug fixes and you can have more confidence with doing a live update.
Hope this helps.