It was AD&D to 3E and I finally found a reasonably readable copy:
http://www.adnd3egame.com/dnd.htm
http://www.adnd3egame.com/documents/conversionbook.pdf
Man, what a blast from the past.
Let me first say I had forgotten about much of it. Except its overall uselessness.
It does try to summarize changes. But all it does is make my eyes glaze over. More than ever, I'm convinced I will never have the energy to use one.
Sure, the first ten or so (of 13) are exactly that kind of incredibly long-winded copy of my three-step procedure, but it does contain what I asked for upthread: magic item and spell equivalencies.
But.
It's a realization that just came to me... I now understand any conversation book like this will always fail, since they ask me to make a thousand little edits in the rules of my shiny new edition. That's like manually patching a computer game: not gonna happen.
Perhaps I'm just older by now, but what I would need is a separate edition of the game with all the little patches already entered, so I get a book that looks and reads as a whole. No references. The full text added in-sentence.
Full disclosure: 15 years ago I would have been all over the 3.5 to PF guide, obsessively discussing every atom of the procedure.
Besides, in what direction is a conversion guide useful
anyway?
Like has already been stated upthread, converting player characters is the easiest part (at least if you use my three-step procedure...) and also, that most of us are past this step many months ago.
Just lists of which items or monsters translate to other items or monsters are way too work-intensive. I'd rather just go on experience and intuition and creativity.
What I would need nowadays, I realize, is a custom-made PHB, that uses all the rules language from the module I'm about to run (say an AD&D or 3.5E one) but nevertheless results in a 5E compatible game.
Meaning that I can read about the Harp of Awesome Power and get the effects from its 5E-balanced counterpart, whatever musical instrument that might be, without having to make any lookups or even realizing 5E doesn't even have Harps of Awesome Power.
I'm getting too old for this shít.