I am planning on replacing the worn 2x6 boards on my deck. The deck
joists are 24" apart. Is there any advantage in using the 5/4 deck
boards over the 2x6's? Obviously if I use 5/4 board I will have to add
an additional joists.
Also I thought of maybe flipping the existing 2x6's to the good side. I
did one and it looked pretty good. But I don't know if this is only a
short term solution.

For turning over, If the deck was done right the crown of the grain is
up so the boards dont cup and hold water, look at the ends. If wood is
not rotted cleaning and stain may be all you need, all treated cracks
but looks good stained.