No, they weren't all concocted long ago. There were no machines to re-purpose long ago, Megatron came to Earth seeking the allspark. He never actually said what his plan was, that was the Autobots concluding what he could do with it. The plot of RoTF was attempted after the events of 07. The plot of DoTM obviously had to be scrapped when Sentinel disappeared, and was attempted again after Soundwave discovered the Ark on the Moon. Your talking about plans changing because of events forced them to. As well, there is no reason why the Decepticons can't harvest the Sun after they have already conquered it.