The book actually provides an answer to this question; I happened upon this section while trying to figure out the answer to a different question.
STEP 7: DETERMINE ONGOING EFFECTS
If you sustain the spell (that is, if you want the effect of a Sustained spell to continue), you take a –2 dice pool penalty to all tests while it is sustained. You can sustain multiple spells, but the –2 penalty is cumulative for each spell. Area effect spells that are sustained can be moved as a Complex Action, provided that the spell area is in the magician’s line of sight. Characters that drop out of the affected area are no longer affected by the spell, while characters that are enveloped by the new affected area must defend against the spell accordingly.
There's also this specifically for Detection spells.
Counterspelling may be used to defend against active detection spells, even if the target is unaware of them being cast (for example, if they walk within range of an active Detect Enemies spell).
That should settle the question of moving AoE spells, I think.
Still doesn't answer the OPs question, but I stand by my interpretation that unless specified like for Hot Potato a magician casting a spell can choose who the targets are in an AoE unless it's a physical effect outside of the magicians control. This would give a slight advantage to direct combat spells as you could choose to cast a mana ball on your opponents even if one of your team members is actively engaged in close combat with them without fear of hurting your team mate.
This interpretation also makes sense for spells like Silence, where a magician could choose to not affect his crew so that they could converse within the sphere of silence while anyone else would be affected normally.
ETA:
To clarify, I think the only spells where a magician doesn't get to choose who is affected are indirect combat spells, as the magician only shapes the source of the spell and lets the mana do the rest.