A Prevenience card (Marauding Dilemma) is the first Magic card to allow the mode to be changed when a spell is copied.
A new Rule 415.8 (Changing Modes) is added to the Comprehensive Rules and a new term "Change a Mode" is added to the Glossary.
415.8a The mode of a spell or ability can change only to another legal mode and only if a card specifically permits a change in mode (such as Marauding Dilemma from Prevenience). If the mode can't be changed to another legal mode, the original mode is unchanged.
415.8b Modal spells and abilities may have different targeting requirements for each mode. Simply changing a spell or ability's mode can't change its targets, unless a card also specifically permits a change in targets. If a mode can't be changed without changing a target, but the card doesn't specifically permits a change in targets, the original mode is unchanged.
Change a Mode
The mode of a spell or ability can change only to another legal mode. If the mode can't change to another legal mode, the original mode is unchanged. Simply changing a spell or ability's mode can't change its target. You can change the mode of a spell or ability only if an effect tells you that you may change its mode. See rule 415.8, "Changing Modes."