Since the previous answer quoted Josephus, I would like to draw on this reference also: (Book XII, Chapt. 5:1 (Loeb 12:241) Antiquities of the Jews)
And the sons of Tobias took the part of Menelaus, but the greater part
of the people assisted Jason; and by that means Menelaus and the sons
of Tobias were distressed, and retired to Antiochus, and informed him
that they were desirous to leave the laws of their country, and the
Jewish way of living according to them, and to follow the king's laws,
and the Grecian way of living. Wherefore they desired his permission
to build them a Gymnasium at Jerusalem. (15) And when he had given
them leave, they also hid the circumcision of their genitals, that
even when they were naked they might appear to be Greeks. Accordingly,
they left off all the customs that belonged to their own country, and
imitated the practices of the other nations.
This is the procedure best described as "καὶ (and) ἐποίησαν (they-DO/MAKE-ed) ἑαυτοῖς (selves) (dat) ἀκροβυστίας (foreskins) (acc)" which is quoted from 1 Maccabees 1:15 (KJV):
And made themselves uncircumcised, and forsook the holy covenant, and
joined themselves to the heathen, and were sold to do mischief.
This is very important distinction: it is one thing to 'hide' one's circumcision with a suturing procedure, versus an "epispasm", to which if we follow the links, requires the 'tissue' removed at circumcision to be replaced; something that was medically impossible until the 20th century.
What one must derive from the meaning of Paul's statement is
"περιτετμημένος τις ἐκλήθη μὴ ἐπισπάσθω"(Having been circumcised-anyone was called, not, let him be uncircumcised)
The key to Paul's understanding is the following sentence,(vs 19)
"Circumcision is nothing, and uncircumcision is nothing, but the
keeping of the commandments of God."
If one, by faith in keeping with their Jewish heritage, becomes circumcised, then keeps the Law, his circumcision is in obedience to God. Rom. 2:25 states,
"For circumcision verily profiteth, if thou keep the law: but if thou
be a breaker of the law, thy circumcision is made uncircumcision."
Therefore, according to Paul, not to obey the Law is the same thing as being uncircumcised.
Therefore if the uncircumcision keep the righteousness of the law,
shall not his uncircumcision be counted for circumcision?
We don't need to ask someone to drop their trousers to tell if they are circumcised or not; those that obey the Law, according to Paul, are "circumcised", whereas those who act unrighteously and disobey the Law are not.
Since the "Hellenizing" period of 170-145BC, when the Hasmonean Priesthood established their Dynasty; there has not been a widespread movement to 'hide' one's circumcision, in fact the Zealots along with the Essenes re-established the practices of the Law, and punishment for those who disobeyed it. (Taken from Wikipedia.) Therefore, given the context of Paul's statements, along with the rest of his writings concerning circumcision, it must be understood that:
1) You would have to wait until the 20th century to be 'uncircumcised', the best you could do was to 'hide' your circumcision.
2) Paul's understanding, communicated from the particular passage and various other passages was that "circumcision" was obeying the Law. To be "uncircumcised" was to not obey the Law, regardless if one were circumcised or not.