Yes 2 Core on the same PC. But I have 2 users on it. Both of them are me, one user is like "for serious business", another one is like for "things other than work".
So these 2 Core belong to two different users and can run independently.
Okay, that explains it.
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So it doesn't want to launch properly just with the wallet.dat that "doesn't belong to it" :-)
Can one draw conclusion that this method doesn't work?
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UPDATE: It worked!!!! I'll write later how I did it!
But I was waiting over 2 hours until Core launches and start running. I didn't try to shut it down (because it takes so long time to launch), I was just waiting.
And it worked.
After it launched, the right wallet with right balance and history was shown.
It just had to rescan the blockchain for the newly replaced wallet.dat,
since the newer Bitcoin Core has "
fresher" blockchain, it took a while to reach the tip.
As a bonus, your old wallet.dat's balance appeared since its transactions are probably within the older blocks that the newer Core has.
For comparison, rescan will take way more hours than that if your Bitcoin Core is fully synced.