OO7 was never in the SAS.
As I've pointed out before, there are a couple of passages where Fleming starts talking about how when Bond was in the RNVR (Navy) he was off behind enemy lines shooting a bazooka in the middle of the Ardennes. Now, admittingly, I'm not too knowledgeable of the British military etc, but a guy in the RNVR in the middle of the Ardennes sounds like rubbish to me. The point here is that one could probably contend that Bond was in some sort of special forces (given Fleming -- SOE) from day 1. If you update that for today, it probably would be the SAS. Doesn't that make more sense? I don't really see a guy making a jump from the RNVR to the 00 Section, but joining as RNVR, becoming SOE, and then a 00. That's logical. To me anyway.
After becoming a 00 he works with the SAS and the SBS per Gardner, which I don't think would be out of the ordinary.
As Zencat has pointed out. It could very well simply be a scipt point that describes Bond as being an SAS type.
Well, we just don't know (obviously). But, still, I get the feeling that Broccoli and Wilson really want to create "their" James Bond, so to speak. I seem to remember someone pointing out here on CBn a while back how curious it was that there was so much new "backstory" for 007 in GOLDENEYE (friendship with Trevelyan, etc.). And GOLDENEYE increasingly seems like a dry run for the full-on fasten-your-seatbelts-this-is-the-big-one reboot that is CASINO ROYALE.
I recall a Purvis and Wade interview - forget where I read it, but I think it dates back at least to the time of DIE ANOTHER DAY - in which they pointed out that the James Bond of today could not possibly be the Bond of Fleming or the early films, not for reasons of suspending disbelief at the guy's inability/refusal to age, but because the Bond of the Brosnan films was clearly too young to have been brutalised by the Second World War.
If I remember correctly, P&W said that, because of this, they had created their own backstory for ModernBond, to explain how someone of his generation could have turned out that way. Y'know, brutalised and stuff. Indeed, I think they even commented that "their" Bond would have had to have joined the modern services, off his own bat, so to speak, thereby becoming a warrior in "peacetime" (my memory may be playing false, though).
Of course, this is something P&W did purely to aid them in their writing, and we haven't seen it onscreen in the finished product.
Yet.