You are right in identifying the change in tense from the past to the present in this thought-provoking story as one of significant importance. It is important to remember that the author is asking us to consider the existence of a hypothetical world. She is eager to "convince" us of its veracity, even though she adds and takes away elements of Omelas, seemingly at whim, to make it more believable:

O miracle! but I wish I could describe it better. I wish I could convince you. Omelas sounds in my words like a city in a fairy tale, long ago and far away, once upon a time. Perhaps it would be best if you imagined it as your own fancy bids, assuming it will rise to the occasion, for certainly I cannot suit you all.

It is after a series of such imaginings, when the author plays with the idea of adding orgies, drugs and soldiers to her description of the wonders of Omelas, that we suddenly switch to the present tense, as if to persuade us even more that the author is describing a real city and to convince us of its existence. Whilst the description was in the past, we were free to treat it as the author fears we would interpret it: as a fairy tale or a myth. However, bringing the action and description into the present tense gives it an urgency and a reality that is more difficult to ignore, which of course leads perfectly to the description of the one glaring anomaly in this description of the miraculous city, forcing us to sit upright and consider Omelas not as a hypothetical entity but as a potential reality.