Do I understand it well: if I will now disenchant all the cards I have from the core set - will I get them again for free when the patch will go live?
You cant disenchant the cards from core set , they are not idiots to give away free dust like that. When the new core set is online you will lose access to the cards that are rotating out and gain access to the new ones
Note if you can de cards that are in the coreset then that means you actually own the card cause all core set cards can be opened from packs . So that means you wont be able to use that card in formats that you could
You get all non-disenchantable core set cards FOR this year. If you dust your copies and they leave the core set you won’t have them for wild/classic/duels/tavern brawls etc. Normal rules apply, only dust to craft and start with cards you dislike the most.
As others have said, Core cards cannot be disenchanted.
Many Core cards exist ALSO in another set. Depending on what set that is, it may be possible to craft (of you don’t own it) or dust (if you do) that other copy. As long as the card is in Core, having that extra copy ONLY affects whether you can use it in Classic game mode (if it’s one of the cards from the original first set).
When the card is no longer in Core you lose the “free” copy and that other copy becomes your only copy … if you have it. From that point on, having or not having that (now only) copy determines whether the card can be used in Classic as well as in Wild.