Paul is speaking figuratively. It's not possible to uncircumcised someone, as you have figured out. The point is that a God doesn't have rules for being in his family. He offers salvation as a free gift, and anyone is welcome to come and benefit from it. You don't have to become a Jew (circumcision and the Law), you don't have to swear off your Jewishness, you don't have to do ANYTHING except come to God with empty and open hands. Christianity offers freedom.

First Corinthians chapter 7, verse 18 reads: "Was anyone at the time of his call already circumcised? Let him not seek to remove the marks of circumcision." (ESV)

My question is how is that even possible (and why bother)? This suggests to me some sort of cosmetic surgery, carried out in Paul's day, wherein one could make it appears as if one was not circumcised. Was there an actual procedure to do that? And if there was, why trouble oneself because who would be seeing one's private parts anyway? Perhaps one's wife, but surely she would know if her husband was of a people who are generally circumcised in their early youth. So who would one be fooling?