Can the sorcerer's ability be used at any time during the turn?
From the rules

Once per turn per opponent, your Sorcerers can conquer
a region by substituting 1 of your opponent’s active
tokens with one of your own from the tray.

From the FAQ

Q: Can Sorcerers use their Ability after placing all their units?
A: It depends what you mean. The Sorcerers cannot use their Ability after doing a final conquest for the turn using the Reinforcement die roll. But they can use their ability otherwise.

So it should be playing during the attack phase, before the reinforcement die roll.
Personally, I don't think it makes a big difference, but in Smallworld one point can make the difference.