3D conversion didn’t take that long, even in 2009. I don’t think they were working on 3D conversions that early, either. I think it’s much more plausible/probable that they got the idea to do 3D re-releases in 2011 (maybe late 2010), and decided that in order to make the 3D conversion as good as it could possibly be, they’d go back to the 97 negatives and re-scan everything in 4K (I believe the new scans were created in that year). Since the blu-rays were already more or less completed by that time, it was easy to take those changes and apply them to these new scans, plus add some extra changes that hadn’t gone into that home video release.

Essentially, it seems like the 3D re-release plan was the catalyst for Lucas to essentially start over from scratch in creating his preferred versions, the release of which would act as a guaranteed 3D income stream as well as promotion for whatever his sequel trilogy was going to be at that time - this is essentially the exact same purpose the Special Edition restoration served for his Prequel Trilogy: Sell the “old” versions on home video (1995: THX releases, 2011: blu-ray releases) screen “new” versions in theaters (1997: Special Editions, 2012: 3D re-releases) to generate hype for the new trilogy coming soon (1999: PT, 2014/15: ST)

And then TPM 3D came out, disappointed everyone financially, and the conversation surrounding the release was mostly negative. The 3D re-release idea was scrapped, and shortly afterwards he decided he was just going to sell the company to Disney.