With respect to the textual variation between Greek texts of 1 Cor 10:9 (some read χριστον [Christ] while others read κυριον [Lord]), we read the following note in the NET:
Χριστόν (Christon, "Christ") is attested in the majority of MSS, including many important witnesses of the Alexandrian (î46 1739 1881) and Western (D F G) texttypes, and other MSS and versions (Ψ latt sy co). On the other hand, some of the important Alexandrian witnesses have κύριον (kurion, "Lord"; א B C P 33 104 1175 al). A few MSS (A 81 pc) have θεόν (theon, "God"). The nomina sacra for these readings are quite similar (χν, κν, and θν respectively), so one might be able to account for the different readings by way of confusion. On closer examination, the variants appear to be intentional changes. Alexandrian scribes replaced the highly specific term "Christ" with the less specific terms "Lord" and "God" because in the context it seems to be anachronistic to speak of the exodus generation putting Christ to the test. If the original had been "Lord," it seems unlikely that a scribe would have willingly created a difficulty by substituting the more specific "Christ." Moreover, even if not motivated by a tendency to overcorrect, a scribe might be likely to assimilate the word "Christ" to "Lord" in conformity with Deu 6:16 or other passages. The evidence from the early church regarding the reading of this verse is rather compelling in favor of "Christ." Marcion, a second-century, anti-Jewish heretic, would naturally have opposed any reference to Christ in historical involvement with Israel, because he thought of the Creator God of the OT as inherently evil. In spite of this strong prejudice, though, Marcion read a text with "Christ." Other early church writers attest to the presence of the word "Christ," including Clement of Alexandria and Origen. What is more, the synod of Antioch in A.D. 268 used the reading "Christ" as evidence of the preexistence of Christ when it condemned Paul of Samosata. (See G. Zuntz, The Text of the Epistles, 126–27; TCGNT 494; C. D. Osburn, "The Text of 1Co 10:9, " New Testament Textual Criticism: Its Significance for Exegesis, 201–11; contra A. Robertson and A. Plummer, First Corinthians [ICC], 205–6.) Since "Christ" is the more difficult reading on all accounts, it is almost certainly original. In addition, "Christ" is consistent with Paul's style in this passage (cf. 1Co 10:4, a text in which Marcion also reads "Christ"). This text is also christologically significant, since the reading "Christ" makes an explicit claim to the preexistence of Christ. (The textual critic faces a similar dilemma in Jud 5. In a similar exodus context, some of the more important Alexandrian MSS [A B 33 81 pc] and the Vulgate read "Jesus" in place of "Lord." Two of those MSS [A 81] are the same MSS that have "Christ" instead of "God" in 1Co 10:9. See the tc notes on Jud 5 for more information.) In sum, "Christ" has all the earmarks of authenticity here and should be considered the original reading.
Why is this significant? If “Christ” and not “Lord” is the original reading of 1 Cor 10:9, it would show that, in the Christology of Paul, Jesus personally pre-existed His conception, contra Socinian and other errant theologies. Furthermore, it would show that a “high” Christology is very early and not a later development, contra much of New Testament/early Christian scholarship. Furthermore, it would also explain why, speaking of the “rock” (πετρα) that followed the Israelites, Paul did not say it “represented” Christ, but that it was (ην) Jesus, providing further evidence that, in the Christology of Paul, Jesus personally, not merely notionally, pre-existed.