Philotomy Jurament
First Post
I view the rules (especially the AD&D initiative rules) as guidelines for the DM to best adjudicate the situation. So my answer is "it depends."Are enemies in fact allowed to do this?

I wouldn't let the enemies change their charge into a tactically superior option, but I might make it a "morale issue," especially if the distance took more than a few seconds to cover. For example, if the enemies charge, and there's time for them to see the PCs present a line of braced and glittering spears, I might roll for morale to see if the charge disintegrates in confusion, or if some individuals drop out.
If the enemies charge, and the PCs shout "we offer you tribute for passage," and there's sufficient time/distance to halt the charge, I might rule that the enemy commander orders a halt for further negotiations.
With the AD&D initiative rules and determining who does what when, my advice is to go with your gut and what makes sense, using the rules as a guide. The general rule is that 1d6 is rolled and high roll goes first. Everything else is a collection of exception cases for that general rule. Keep in mind that a round is a minute and the abstract nature of combat, and think about what makes sense. Can they halt the charge in time? Can the archer shoot before the charge covers that ground? Et cetera.