PTO doesn't have a data export function.  Unofficially, however, you can call Intuit Support and request for the data in Lacerte format, which you can then convert for PS.  It is a manual process and they could usually get it done within a workweek.

As you may be aware, Intuit Link was fully integrated into PTO, if I recall, two seasons ago.  It has since been accessible only from within the Link drawer of PTO.  The legacy Intuit Link portal is no longer operational for PTO users, although it is still the portal for Lacerte and PS users.  If you log into the legacy Intuit Link portal, you will notice that none of the Questionnaires/Checklists or discussions you have with your clients since the migration are present or visible (although you still have access to data for those pre-migration years (sans item level comments).  You may not have noticed but even the client portal for PTO users was migrated; the client portal for PTO is prolink.intuit.com whereas it is link.intuit.com for everyone else.

In practice, what this means is that you will need to send brand new invitations (with new tokens) to your existing clients who will log into the legacy portal once you move onto PS (i.e. link.intuit.com).

[Edit: I meant to say "Unofficially" instead of "Officially"]