The version of "Gyro Ball" attack on Forretress says "you may switch..." as an optional action, but the version of "Gyro Ball" on Ferrothorn indicates the switching part of the attack is mandatory. Which way is correct?
The effect may vary from card to card, so you should play each attack as it is written on its card. So for Forretress the switch is optional, but for Ferrothorn it is mandatory.
Source: Emerging Powers FAQ (2011-09-01), PUI Rules Team (2011-09-01)

Relevant cards

Note: All applicable cards may not be listed below.
BW2_72
DP7_6
HGSS3_3