Is it possible to convert a multi question form to a single question form without losing data already collected?

Yes, it's possible. You can change your form layout from your form settings.

But I checked your account and I found that you are using mixed layout format(both single question and multi question), so you won't be able to get this option under your form builder.

You can sen us your form link which you need to change from multi question layout to single question layout and we will do that for you.