My family comes from an area where there are lots of Thigpens, may even have some in the family aways back.
That said, I have done some study of phonetics and the rules of phonetic shifts. I can see Bibben, Bippen, Vippen, and so on but not Thigpen which would give Digpen, Tigpen.
How do you explain the shift from p to g? Impossible. G usually shifts to k or hard c.
Furthermore, place your tongue in position for the f sound and then for the th sound. You will appreciate how hard the shift is.