The rules on timing are not clearly spelled out, but the new spell does need to be different from the first. You're not really changing the bound spell if you're keeping the same bound spell. You'd be changing the orientation, but not the spell, which I think is clear is what they meant. This meaning of the word change is also corroborated on page 21 of the rules book where it specifies changing the bound spell to a different spell.
Currently the lack of clarity on timing doesn't really have any effect on game play and can be put off until it becomes relevant.