Currently (to my knowledge), there no easy way to upgrade or migrate an older Survey123 Connect Excel form from an older version of S123 to the most recent version. There have been some releases of S123 Connect that significantly change not only the look, but also the schema/functionality of the Excel S123 form in Connect.
Currently, the only way to "upgrade" the form is to manually copy and paste the values in the Excel form from the old version of the form to the new version of the form. This is a slow and tedious process because Esri sometimes changes the column ordering, so you can't copy and paste row by row. You have to copy and paste column by column.
I would like to see some functionality that would allow users to upgrade an existing form from an older version of the Survey123 form to a new version. Or alternatively, migrate an older version of the form to a new version.