I've had to change the order of questions before in past surveys.
Your existing data is fine. The data is captured. So no issues there.
Regarding your new surveys, they will be captured according to the new survey order. The same data will be captured as per the old survey version, with just the order being different. We are still dealing with the same questions right?
The concerns are with the incompleted surveys when you make the update. If for example you had a question at the back and you moved it to the front, an incompleted survey half way through the survey that recommences will continue from the half way mark and go through to the end and will miss that moved question.
Providing it is just the movement of questions and not changing question structure (e.g. a multiple response question becoming a single response question), I have found it to be okay.
I certainly don't encourage the movement of questions, but if it needs to be done, so be it.
I encourage the following before making the changes ...
1/ Backup your current data.
2/ Take a snap shot of the incompletes counts in the admin module.
And if you want to go one step further, do a test survey yourself and suspend it half way through, then update the survey with the moved question/s, then recommence that suspended survey and see the effect.
Hope that helps.