I think the idea of changing the set times by taking 15 minutes from #4 and giving it to #2 is a good idea in this situation. Repeat acts are not a problem, and even a good thing, as several have been well received in the past. Repeat headliners are certainly a good idea if they went over well the first time, as long as there's been some time between their last appearance and they have some new material out.
As for lower-slot repeats... well, as some have pointed out, there are a few previous 3rd slot bands, for example, that probably couldn't have headlined the years that they were playing, but did so well during those performances that the ProgPower audience would love to see them return as headliners.
I don't know if it really makes sense for a previous opening band to return as an opening band though, unless they have released some new albums since then and the buzz on them has jumped up and people's perceptions are different. I think the idea of being an opening band is to build, partially by using the bigger name of the headliners to draw an audience to display their skills. If they come back just to play the same opening slot and get the same crowd reaction, there seems little point... because while there may not be that many true headliners that haven't played yet, there are plenty of new, lesser-known prog and power bands that people would love to see in an opening slot.
As for lower-slot repeats... well, as some have pointed out, there are a few previous 3rd slot bands, for example, that probably couldn't have headlined the years that they were playing, but did so well during those performances that the ProgPower audience would love to see them return as headliners.
I don't know if it really makes sense for a previous opening band to return as an opening band though, unless they have released some new albums since then and the buzz on them has jumped up and people's perceptions are different. I think the idea of being an opening band is to build, partially by using the bigger name of the headliners to draw an audience to display their skills. If they come back just to play the same opening slot and get the same crowd reaction, there seems little point... because while there may not be that many true headliners that haven't played yet, there are plenty of new, lesser-known prog and power bands that people would love to see in an opening slot.