The closest I have to regret is I bought most of the IDM/ambient house stuff of the ‘90s on CD rather than vinyl. I suspect that cost me around £8k or so in collection value! As an example my copy of Blue Lines os a CD, but it is the first issue just branded ‘Massive’ as the phrase ‘Massive Attack’ was apparently banned due to the Gulf War. I’m not bothered about swapping this stuff out as much of the reason I switched to CD was ‘90s pressings were so piss-poor and noisy. I went through about three copies of the first Orb album trying to find a quiet one so bought the CD. I’ve got the limited triple of the second but at least one side is hopelessly eccentric. Much 90s vinyl was as bad as it ever got despite the crazy high value now.
Most other stuff I’d missed at the time I’ve back catalogued over the decades, e.g. I’ve got orange RCA label originals of The Idiot, Lust For Life, TV Eye, though chances are I’ll never find nice 1st pressings of the first and Fun House.
I’ve sold and rebought stuff and always view my collection as fluid. A lot comes in, a lot goes out. I’m struggling to think of stuff I regret not buying, as most of it has turned up since. I‘d like more Throbbing Gristle originals (I’ve only got 20 Jazz Funk Greats, the rest are reissues), and there’s an obvious gap where The Pop Group’s ‘Y’ should be. I’d prefer my Prince collection to be on vinyl, but I’m not that bothered. The CDs are fine. Jazz vinyl is my favourite vinyl, and I’m just not old enough to have bought that new. I started with Miles Davis Tutu and a Blue Note sampler in the mid-80s, and given that late start point have amassed a really nice collection with a fair number of originals in it.