Made up a test form for you and did this two different ways.
My dropdown has options of 1, 2, or 3
Option 1 in the dropdown OnChange I placed
Switch(Self.Selected.Value,
"1", Set(varDrop,"Option1@Gmail.com"),
"2", Set(varDrop,"Option2@Gmail.com"),
Set(varDrop,"Option3@Gmail.com") )
then in text or label of another datacard i placed default as varDrop (if it does not appear as you change dropdown selections be SURE that it is text/number set to same data type as the column the datacard is for)
this method chanegd the default value in the control every time i swapped dropdown options, works great
another way in another datacard i put another label and gave it the default value of
Switch(Dropdown2.Selected.Value,
"1", "Option1@Gmail.com",
"2", "Option2@Gmail.com",
"Option3@Gmail.com")
and this also achieved same results, so two options for you. Take care 🙂