There is no discrepancy. "Until" doesn't itself denote a change after the given period demarcated. That is to say, it doesn't itself contain information about what follows. Its context could imply There may be one (e.g. by the obviousness or necessity of there being a change afterwards), but the 'until' doesn't itself carry that information. It is a simple demarcator of a limited time period in view: here 'the time until all enemies shall have been brought under Him.'
A famous historical example is Matthew 1:25:
Matthew 1:25 (DRB) And he knew her not till she brought forth her firstborn son: and he called his name JESUS.
That is, some use the argument that since Matthew wrote that Joseph did not have relations with Mary "until" Jesus was born, that therefore he did afterwards. In the words of the great Jerome, "I don't know whether to laugh or cry!" Whether he did or did not have relations with Mary after the birth of Jesus, this "until" doesn't carry or even imply such a course of events, since that would make the focus not on Joseph's role in the conception of Jesus, and Mary's actual virginity (the purpose of Matthew's including this little comment), but about the 'sex life,' to use profane terminology, of Joseph and Mary, which is irrelevant to Matthew 1's context apart from showing what role Joseph had in the conception of Christ.
I don't think most people think through what this reading of 'until' (as indicating a change afterwards). It would read like 'Joseph didn't know Mary before Jesus, but did after; and by the way, he called the child Jesus).'
The humble 'until' simply never carries such information. A change must be inferred from somewhere else.
Also, Ephesians doesn't say anything about Christ's reign (or in any event, the length thereof) but that His titles and names and power is far above any creature, celestial or terrestrial. It also does not say that He has this kind of reign or role forever in "the age to come," (which is a Biblical term for eternity or heaven, not a future place or thing absolutely, but relatively to time) but rather there will be place for whatever it means for God to be "all in all," which I believe has to do with the fact that the Son in His humanity must lead people to God until "the fulness" come in, and that the people of God cannot have as the center forever a man, or rather a human nature, but God Himself. Jesus' mission and role in the Trinity is to herald and bring all to His Father—even in His divine nature (since it doesn't imply subservience to serve the other in love, especially when that is perfect, selfless, divine love), even if that means bringing people to recognize that in Him the Father is fully present, and that He is the very image and expression and speech of God Himself, nothing less. But the Speech must also of necessity come back to a Speaker. I think this is what God being "all in all" means.