If I play "Giovanni's Scheme" and choose one of its effects, can I change to the other effect later in my turn as long as I haven't attacked yet?
No. You choose which effect to use when you play it and you're locked in with that effect until the end of your turn.
Source: BREAKthrough FAQ (2015-11-19), TPCi Rules Team (2015-11-19)

Relevant cards

Note: All applicable cards may not be listed below.
XY8_138
XY8_162