Guide
Linking Pokemon HOME to Champions
Your Pokemon are visiting Champions, not moving in, and the first time you collect one its strength is locked in. The rules below are quoted from the game itself, along with the four Mega Stones you get for linking.
Get the Pokemon right before you collect it
When you collect a Pokemon that is visiting this game from Pokemon HOME for the first time, that Pokemon's current strength will be used to determine its strength in Pokemon Champions. From then on, even if you make changes to that Pokemon in other games, some changes will not carry over to Pokemon Champions.
Pokemon Champions in-game text
In practice: whatever state a Pokemon is in the moment you first collect it is the version Champions keeps. Going back to another game to train it further will not fully update the Champions copy. Champions also has its own 66-point training system, so most of what matters competitively is decided here, not brought with it.
Rewards
Four Mega Stones for linking
Linking sends four Mega Stones to your mailbox as part of a Pokemon Legends: Z-A campaign. They cover the three Kalos starters and Floette. Worth knowing: every one is also on sale for 2000 VP, so linking saves you time, not access.
Questions
Pokemon HOME transfer, answered
How do you link Pokemon Champions to Pokemon HOME?
Link the two from inside Champions, then use Collect from Pokemon HOME. Your Pokemon arrive in a Visitors’ Box and stay yours to send back whenever you want. They are not moving in. The game puts it plainly: "To bring your Pokemon to Pokemon Champions, you will need to link your game to Pokemon HOME."
Do Pokemon transfer permanently to Champions?
No. They visit. Champions keeps them in a Visitors’ Box and Send Back to Pokemon HOME returns them whenever you want. You cannot send back a Pokemon that is currently entered in an ongoing competition.
Does a transferred Pokemon keep its stats from other games?
This is the part that catches people out. The first time you collect a visiting Pokemon, its strength at that moment is what determines its strength in Champions. After that, changes you make to it in other games will not all carry over. Get the Pokemon how you want it before you collect it.
What do you get for linking Pokemon HOME to Champions?
Four Mega Stones, sent to your mailbox as part of a Pokemon Legends: Z-A campaign: Chesnaughtite, Delphoxite, Greninjite, Floettite. They unlock the Kalos starters’ Megas plus Mega Floette. All four are on sale for 2000 VP each anyway, so the gift saves you 8000 VP. It does not unlock anything you could not buy.
Can you use HOME Pokemon in ranked battles?
Only if they pass the same legality checks as any other team member. Champions rejects teams containing an ineligible Pokemon, too many restricted Pokemon, a disallowed move, or a disallowed held item.
Everything on this page is taken from Pokemon Champions' own in-game text. Champions runs a curated 396-form roster, so a Pokemon in HOME is only usable here if its species is on that roster.