I'm confused! Please help me...

I need to book some seats from MCO-BOS later in the year. Obvious choices are Song vs Jet Blue. Never used either before, but now I see that song is being incorporated back into Delta... but they're adding FC seats to the song planes... but these planes are then going to fly delta mainline routes or something according to the rather confusing letter posted here.

Does this mean that a rather ropey old Delta plane will be flying the Orlando routes or what - I just can't get my head around the deal...

Any explanation greatly received!

Thanks

Tony