"Core" refers to the essential rulebooks: Pathfinder Core Rulebook for Pathfinder 1st Edition, Pathfinder 2nd Edition Core Rulebook for Pathfinder 2nd Edition before the Remastering, and Pathfinder Player Core, Pathfinder Player Core 2, and Pathfinder GM Core for Pathfinder 2nd Edition Remastered. The word "Core" does not narrow down which edition you want to convert to.
I have converted the Dungeons & Dragons 3rd Edition (D&D 3.5) version of the Rise of the Runelords to Pathfinder 1st Edition (PF1) and I have converted the PF1 Ironfang Invasions adventure path to pre-Remastered Pathfinder 2nd Edition (PF2). I am currently running the PF2 Strength of Thousands adventure path under a mixture of PF2 and PF2R rules, because they are so similar that they can work side by side.
Though Pathfinder 1st Edition was nicknamed D&D 3.75, converting Rise of the Runelords from D&D 3.5 to PF1 was more than I could handle alone back in 2011. I relied heavily on material created by other fans who had done the conversion. Just as the campaign ended, Paizo did their own conversion in the Anniversary Edition of Rise of the Runelords, but I never compared the two separate efforts.
I was a more experienced GM in 2019 when I decided to convert Ironfang Invasion to PF2 rather than run Age of Ashes, the only PF2 adventure path published at the time. Most of the conversion was straightforward: swap out a PF1 monster and put in the PF2 monster of the same name. Sometimes, the PF1 monster had not been ported to a PF2 Bestiary yet, so I used a different monster of the same level. But I had to port unique characters over to PF2 myself, using the Building Creatures rules in the PF2 Gamemastery Guide. The PF2 Remastered GM Core has an almost identical chapter, Building Creatures.
The economy and magic items changed greatly, so every treasure table had to be rewritten from scratch.
PF2 Remastered occurred because of Hasbro/Wizards of the Coast's Open Gaming License scandal in 2023, The D&D Open Game License controversy, explained. Pathfinder 1st Edition used a lot of material from D&D OGL 1.1, but Pathfinder 2nd Edition was mostly independent of that license. Nevertheless, it had a lot of names in common. The Remastering was a massive name change to prevent Wizards of the Coast from ever insisting that Paizo pay for the D&D content in PF2. Thus, gripplis became tripkees and gnolls became kholos, and dragons no longer came in the colors of the rainbow. And a lot of spells had name changes. Classic names from folklore, such as elf and dwarf, were safe so they did not change.
Alignment of good versus evil and law versus chaos were a D&D creation and were dropped, which made the alignment-based champions change their causes. The classic schools of magic such as abjuration, conjuration, and evocation were also a D&D creation, so the wizards lost those. Paizo took the opportunity to publish a few power corrections to some weaker classes, but unfortunately the no-longer-powerful PF2 wizards ended up weaker than ever.
But since the Remastering is 95% just a name chance, converting from pre-Remastered to Remastered ought to be a piece of cake.
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There's a number of conversions floating around. You can usually find them on an appropriate reddit, or here, if someone did it for free, but there are others for sale at the relevent webstores as well.
It's not the hardest thing in the world. Some monsters don't have correlaries, but that's pretty easy to work in. The real buggaboos [from my understanding] are effects dealing w/ alignment and treasure. Someone's thrown together a handy DC conversion table from 1E to 2E which is probably a good guide.