So God changed the rules between the OT and the NT.
If I am a landlord of two houses, I can rent them under different leases.
1 Moreover, brethren, I do not want you to be unaware that all our fathers were under the cloud, all passed through the sea, 2 all were baptized into Moses in the cloud and in the sea, 3 all ate the same spiritual food, 4 and all drank the same spiritual drink. For they drank of that spiritual Rock that followed them, and that Rock was Christ. 5 But with most of them God was not well pleased, for their bodies were scattered in the wilderness.6 Now these things became our examples, to the intent that we should not lust after evil things as they also lusted. 7 And do not become idolaters as were some of them. As it is written, "The people sat down to eat and drink, and rose up to play." F25 8 Nor let us commit sexual immorality, as some of them did, and in one day twenty-three thousand fell; 9 nor let us tempt Christ, as some of them also tempted, and were destroyed by serpents; 10 nor complain, as some of them also complained, and were destroyed by the destroyer. 11 Now all F26 these things happened to them as examples, and they were written for our admonition, upon whom the ends of the ages have come. 12 Therefore let him who thinks he stands take heed lest he fall.
In another scripture we are taught that the LAW was a "schoolmaster" used by God to lead us to Christ.
19 What purpose then does the law serve? It was added because of transgressions, till the Seed should come to whom the promise was made; and it was appointed through angels by the hand of a mediator. 20 Now a mediator does not mediate for one only, but God is one. 21 Is the law then against the promises of God? Certainly not! For if there had been a law given which could have given life, truly righteousness would have been by the law. 22 But the Scripture has confined all under sin, that the promise by faith in Jesus Christ might be given to those who believe. 23 But before faith came, we were kept under guard by the law, kept for the faith which would afterward be revealed. 24 Therefore the law was our tutor to bring us to Christ, that we might be justified by faith. 25 But after faith has come, we are no longer under a tutor. 26 For you are all sons of God through faith in Christ Jesus. 27 For as many of you as were baptized into Christ have put on Christ. 28 There is neither Jew nor Greek, there is neither slave nor free, there is neither male nor female; for you are all one in Christ Jesus. 29 And if you are Christ's, then you are Abraham's seed, and heirs according to the promise.
"So God changed the rules between the OT and the NT."
No, He changed the way He changed the way He did business.