Remember while our reading of a text may be as simple as turning our eyes to it, our understanding can often be clouded by modern notions as well as unfamiliarity with the way both writer organized material and the reason a person spoke in such a manner.
There are forms of expression in Hebrew that cannot be understood at first blush or very easily from a translation, especially when one has no familiarity and experience with the culture. To illustrate, not too long ago in American English there was a great difference between calling a woman "phat" and "fat," even though the words sound the same. Someone listening who was not in the loop with fads and US pop culture would not be equipped to arrive at a correct conclusion as to the differences between the two phrases.
While the words in John may be Greek, the original language was Hebrew/Aramaic which uses an indirect way of communication (recall the unique use of the Semitic half-affirmative at Matthew 26:25, 64, 27:11, "you say so" and the "what have I in common that has anything to do with you" expressions of Jesus--a use of the Hebrew denial-of-common-interest form at John 2:4, also used at Mark 1:24, 5:27). These are forms of speech which have no equivalent in English or in our Anglo-American culture. And they are not the only ones that one comes across when reading John, as the verses of 7:8-10 demonstrate.
I came across several commentaries in reference to this reading, and this is the way it is generally explained by translators, theologians, and exegetes:
The Gospel of John relies heavily on a series of word plays that repeat again and again and again. For example, there are several uses of "I AM" that, while they can be translated different ways, are used instrumentally throughout the text in reference to the prologue at the beginning of this gospel.
The same play occurs with the expression "go up." Like Jesus reference to his raising the temple of his body, the gospel of John commonly shows Jesus acting contrary to his words as a way to demonstrate that his disciples often missed the true meaning behind them. Since Jesus does go to the festival, apparently he is once again talking about something which cannot be gleaned from a surface listening to or reading of his words, even in their original tongue. The indirect use of speech common to the Hebrew culture was obviously a pushing-of-the-envelope in the hands of the Nazarene teacher to an extent not experienced before.
Because of this creative use of the indirect-expression in Hebrew culture by Jesus, scholars note that there are three other "go up" statements that occur in John, namely at 3:14, 6:62, and 20:17. All four together are seen as a play on the words "go up", referring not only to going up to Jerusalem but also to exaltation at the cross, the resurrection, and Christ's ascension.
Some early manuscripts have "not yet" inserted that, via analytical textual research has been demonstrated to be an interpolation by those who were unfamiliar with Jesus' original meaning, and took the reading as false the way it stood.