Changing the appearance of a text question to multiline will not affect existing data. Early (beta) versions of Survey123 Connect did delete data on all republishing; if you get a warning message about data being deleted, please download the latest version of Survey123 Connect.
Yes, you can do that without affecting the feature service using Survey123 Connect. The particular change you need to do to make the text boxes bigger is to give those questions the appearance multiline.
Hi. This help topic describes in detail the type of changes in a survey that will force the feature service to be re-created, versus the changes that will not, Publish your survey—Survey123 for ArcGIS | ArcGIS