I assume you have two fields: date of birth and PESEL, and the date from the previous request will come as an input parameter to the form and will be mapped to a session variable.
So, first of all, I would add a technical field with a script attached that calculates the date of birth from the PESEL as timestamp.
Then I would create a technical field that will map the first non-empty value from the list: date of birth from previous form and date of birth from PESEL.
Finally, I would map the technical field mentioned above to the date component via “Źródło danych z innego pola” and set an active condition to check if the technical field is empty. Remember to add listening where required