Hi Alexadkin,
Is "customer" a field of Contact entity?
Here is my guess:
The contact is an existing record, once the option set field is updated to "customer", then this person will be regarded as a new customer.
If my guess would be right,
once any field of entity is updated, the "modified on" field will be updated automatically,
then you could retrieve new customers by selecting a range of period.
e.g:
Find contacts who will be modified within 2 days,
and you could add an extra field to be set in the flow to make result more precise.
e.g: a two option field: "Is in XX business process?", by default the field is false, set it to true in the flow.

In a word, we could identify new customers with date time fields.
Please kindly point out if I misunderstand your requirement.
Regards,
Clofly