A few theories I've heard thrown around:
1. That, in fitting parallel sequence, the intro to the first episode parallels the final sequence of the last in that "all the stars are our enemies" and "all the stars are our friends" show dichotomy and highlight the dramatic shifts that occurred throughout the series. Mainly to sounds artsy.
2. The time between Simon and Nia's wedding ceremony and the wonderer Simon we see at the final sequence suggests, possibly, that Simon's adventures in space may not have ended after the anti-spiral's defeat. This gives GAINAX some room to work with in future installments.
3. Finally, and least likely, is that the Garlock Simon is actually a son, or grandson, to the Simon the series ends with.