Been in this exact situation, kind of. I was until a couple of months ago, an SD only person, but in March I got upgraded to HD.
Thing is my collection is half review discs and half my own purchases, so the spending issue isn't as big a deal. But the first thing I did was buy a wodge load of HD anime, those title that I want in the best possible quality.
Evangelion 1.11 (fixes the subtitle issues with the DVD so very worth it)
The Sky Crawlers
Appleseed Ex Machina
Ghost in the Shell/Ghost in the Shell Innocence
Summer Wars/The Girl Who Leapt Through Time
Full Metal Alchemist: The Conqueror of Shamballa
Paprika
The Wings of Honneamise
As I said, I'm lucky in that I get a lot of review discs, and I'm not a packaging fiend, so I can live with silvers in CD cases, it's what's on the disc that is important. So the only titles I have paid more than once for are... Ghost in the Shell (3rd purchase, sound sync error, corrected DVD, and now Blu-ray). The Girl Who Leapt Through Time, I initially bought the two disc DVD, Paprika, The Wings of Honneamise, bought the R4 DVD.
I've found that a good Blu-ray of an anime film blows the DVD out of the water. For instance comparing the Blu-ray of The King of Eden to the DVD shows only a little difference. But comparing Evangelion 1.11 to the DVD is astounding, and that film got a very good DVD conversion as well.
I actually waited to get the Blu-ray of Ex Machina for the Japanese soundtrack, the DVD didn't have it, so I didn't buy that.
But I find that even with upgrading, I can't ditch the DVDs in many cases. I need Ghost in the Shell because it's better than the Blu-ray. The Blu-ray is only good for 2.0, the original version on it sucks. TGWLTT only has the extras on the DVD. I can probably retire my Paprika and FMA DVDs though.
Anyway, my limited exposure to hi-def anime so far has decided me that Blu-ray for movies and DVD for TV series is the way to go when it comes to my own money. Especially with Manga the sole arbiter of Blu-rays in the UK apparently, and having so much trouble shifting series in that format.
As for review discs, I'll review whatever I get, as always.
EDIT: Come to think of it, I have one HD TV, one Blu ray player, 2 SD TVs, 2 DVD Players, 2 PCs, 2 DVD-Roms. I can't always use the HD TV, so I guess I won't ditch the DVDs just yet, just in case.