Bitcoin Core won’t ever discard reorged blocks, till they’re pruned (which occurs no matter whether or not the blocks are within the energetic chain or not).

Just one chain is taken into account “energetic” at a time although. The foundations are:

• Solely contemplate absolutely legitimate chains (i.e. if a block incorporates an invalid signature or the rest unlawful, it’s ignored, along with all its descendants).
• Amongst these legitimate chains, solely the one whose tip has essentially the most gathered work (roughly equal to the sum of the difficulties within the chain) is taken into account.
• If there are a number of legitimate ideas of equal gathered work, the one whose final block was acquired first is taken into account energetic.

All different chains are thought-about inactive, but when they get prolonged, they could find yourself changing into energetic once more.