Well, to begin with. Many say that "ashes to ashes" is metaphorical speaking; it would be autobiographical, kind of.
Ashes to ashes then, because of his past (that of Bowie) that would be finished (dead), he was turning the page and going clean now.
Major Tom would mean he (Bowie) in the past, so the music would be like he was talking about him in the third person.
The beginning of the music is a bit puzzling (at least for me), Do you remember a guy that's been In such an early song?
it perhaps meant that the song was an "early death" or "tripping" or something else (that time was a bit psychedelic and things hadn´t to make sense)...
I've heard a rumor from Ground Control
Ground control could mean someone from the rehab clinic, or could be that David just meant "rumour has it (that)"...