First of all, make a database update to be on the save side.
In LS5 go to "Configuration" --> "Advanced" --> "Backup entire database" (if this is available in the SaaS offering).
This will guarantee that you don't loose any data. However, you should considering downloading your survey data on a regular basis anyway, in my opinion. LImesurvey is a data collection tool, not a tool to store / analyze data.
Now, if you use any workaround (e.g. via Javascript) or if you have custom themes they might be impacted. Afaik, the themes in LS6 are slightly different to the ones in LS5 and a lot different to LS3, so you might have to make some adaptations.
I would talk to Limesurvey GmbH about it. If you have several surveys running until November, discuss with them what would be a good date to upgrade so you have minimal impact on your surveys (there is never the perfect time for upgrades, but there might be times that are better than others) and maybe you can set a specific time frame for the upgrade. I would try to talk to them about it and see what the response is.
The chance of losing any survey data during the upgrade is minimal, in my opinion. However, it is better safe than sorry. I just now see that you are still on LS3. I was quite sure that all SaaS is already on LS5 by now. However, everything that I mentioned is still valid, but changes are a bit bigger from LS3 to LS6 than from LS5 to LS6.
Help us to help you!
- Provide your LS version and where it is installed (own server, uni/employer, SaaS hosting, etc.).
- Always provide a LSS file (not LSQ or LSG).
Note: I answer at this forum in my spare time, I'm not a LimeSurvey GmbH employee.