Fire Emblem Heroes - April update further detailed

Before Battle Begins, Swap Spaces

You’ll be able to change the starting position of your characters prior to the start of combat. Tap the Swap Spaces button in the menu at the bottom of the screen to make your changes. You can continue to swap spaces as much as you like until a character completes an action in battle.

EXP Increase vs. Weak Enemies

The amount of EXP earned by 1★ to 4★ Heroes will increase in battles against enemies with a lower level than the Hero.
The fewer stars a Hero has, the more pronounced the effect will be.

Maximum Stamina Increase

The maximum amount of stamina will increase to 99. That also means that when you restore stamina using an Orb or a Stamina Potion, you’ll receive 99 stamina instead of 50.