This is a big change to how you can interact with a full board in Legends of Runeterra. It was in the patch notes yesterday, but has been overlooked by a lot of folks.

  • If you have a full board now you can choose to play an additional unit.
  • The old unit is "Obliterated".
  • The old unit does not trigger any Last Breath effects.

Here's the excerpt from the patch notes.

Quote From Legends of Runeterra

Playing Units on Full Boards

  • Having a full board of 6 units no longer prevents you from playing additional units—you may now do so and select a current unit to be Obliterated, with your new unit replacing it.
    • This won't trigger Last Breath effects, because the replaced unit doesn't die.
    • If the new unit you played makes more units after being summoned, those will still "overflow" and be Obliterated—you can't make more space beyond the unit you're replacing. (You can also still play spells that generate units on a full board, but they'll "overflow" too, so maybe don't do that!)

We made this change so that longer games will retain more interesting decisions even late into the game when boards are filling up. We want you to be able to keep playing the sweet units you draw, if you want to, and until this patch, you were essentially locked out if you had a full board of 6 units—you couldn't play any more from your hand. Now, you can replace one of your units on a full board with the new one played from hand—choose the one you want to replace and it gets Obliterated.

Andrew Yip, Design Director, had this to say in a series of Tweets last night.

Quote From Andrew Yip
For global launch, we made a change I wanted to share some more context on. Sorry for surprising y'all with it! Players at max allies (6) will no longer be prevented from playing a unit. Instead, when they try to play a new unit from hand, they'll be prompted to replace one.

We made this change for several reasons:

1) In designing new cards and mechanics, we often found ourselves creating cards that players would trap themselves with. We want to be able to make these designs; they're sweet.

2) We often saw new players discovering the board limit upsetting ways. This was due to trading units less frequently and because the rule isn't well communicated. Other games can support more units or have this 'clobber' mechanic already.

3) While we want skillful play to be king, we also want it to be intuitive and fun for both players. LoL replaced creep deny with more frequent skillshots, brush, and other skill checks. We're confident we can find many more fun skill tests than board limits.