It’s easy to convert a 2D event into a 3D event!
The only difference between a 2D event and a 3D event is this: A 3D event’s behavior depends on the event instance’s position due to 3D space, while a 2D event’s behavior is the same regardless of its position. This means that if you add anything to a 2D event that makes its behavior change depending on its position, that event is technically 3D - and if you remove everything from a 3D event that makes its behavior depend on its position, that event becomes 2D.
The most common way to make an event 3D is to add an FMOD spatializer effect to its master track.
(In fact, the only difference between an event created by the “New 3D Event” and “New 2D Event” context menu items is that “New 3D Event” automatically adds a spatializer effect to the new event’s master track.)
Assuming you decide to make your events 3D by giving them spatializer effects, you’ll need to set each spatializer effect’s max distance to the maximum distance (measured in whatever units of distance your game engine uses) at which you want the event instance to be audible. You’ll probably want to use the same maximum distance for a lot of different events in your project, so you might want to create the spatializer as a preset effect, so that you don’t have to manually set the max distance in every event you’re making 3D.
Another way to make a 2D event into a 3D event is to use at least one built-in parameter in the event, wither to trigger and instrument or to automate a property. This only gives the event the behavior defined by the instruments the parameter triggers or the properties it automates, however, so the event won’t be panned or attenuated based on its position relative to the listener unless you manually create that behavior or also give the event a spatializer effect.