Disguise is one of the most useful tools in a Night's Black Agent's character's kit, but the main book doesn't describe its use in Thrilling Chases. Double Tap does mention the quick change, but doesn't go into detail. Yet the quick change, as described here, is perfect for chases.
So let's talk about three ways in which a quick change can be used.
Swerve. A swerve is a single maneuver that has the potential to change the chase all at once … if either the pursuer or the runner has the higher Maneuver, or if both Maneuver ratings are the same, he can spend 3 points of the chase ability to force a high-risk swerve; all changes in Lead in the the Swerve round are double normal.
Suggested change: he can spend 3 points of the chase ability or 3 points Disguise. If the latter, the Director may ask for a Preparedness check, but is not obliged to.
So in this version the pursuer or runner - more likely runner - opts for a quick change to throw the other party off. This increases Lead, if it works. The typical Lead change in a foot chase is 1 or 2; now it might be 2 or 4.
The Director is within rights to limit this kind of Swerve to agents with 8 or more in their Disguise pool.
Sudden Escape. If the runner has a lead of 7 or better, and wins the exchange of chase ability tests, she can attempt a Sudden Escape instead of changing the Lead. This is something completely outside the parameters of the chase … [and] requires a successful ability test of some kind. The Difficulty of the Sudden Escape test is always 1 higher than the previous Difficulty in the chase.
So in this version the runner uses Disguise as the ability in the Sudden Escape test.
Don't fall into the trap of thinking that a Disguise has to be elaborate. Often the best disguise is something that changes the profile of the runner quickly. Were you last seen bare-headed? Grab a hat, or pull up the hoodie. Last seen in red? Switch to brown, or black. No glasses? Then put on glasses, or sunglasses. Change tops. Baggy sports jackets are perfect for this kind of chase; not only are they memorable, you can wear something completely different underneath. Even changing your gait can help - remember what the Wired video had to say about American vs European stances.
As for new gear, if you have a chance to grab a wig, or new jacket, then by all means do - and this is one of the reasons why the runner always ought to have one eye on her surroundings. If ever there was a moment to dive into a clothes shop, now's the time.
Take this sequence from Baby Driver as your cue:
At about 1.40 he's dashing into the mall, which means two things: plenty of civilians to cloud the issue, and plenty of shops. By 1.50 he's in his first store, and grabbing a new jacket and hat comes soon after. The broken glasses are gone by 2.02. By 2.12 he has a whole new look and is out the door, and it works. That Lead bonus gets him to a car, which since he has the Grand Theft Auto perk is an easy steal, particularly since he used Thrilling Dialogue to grab a useful tool (refresh) on the way out of the electronics store. Several spectacularly failed Driving rolls later he's on foot again, but thanks to a gun-toting partner in crime there's a chance for a Sudden Escape.
I mentioned three ways to use the Disguise ability in a Chase. The Forsythe classic thriller Day of the Jackal has the nameless assassin changing disguises several times - the entire novel is one long Hot Lead chase sequence. One of my favorites comes towards the end, and is remarkably simple. They're looking for an able-bodied man, so the Jackal loses a leg. He also chews on cordite to make himself look ill, an old army trick. That, and a simple costume change plus the appropriate paperwork, gets him through the barricade to his chosen sniper position.
So what's the Jackal doing here? First, he's using Disguise to buy himself some Hot Lead. He immediately spends that Hot Lead to get a chance at the target, and takes his shot. As his Hot Lead is now 0, he faces that immediate confrontation.
Usually it's the agents dealing with Hot Lead, but it doesn't have to be that way. A chase of this sort could be very interesting to play out in an Edom game, with the agents as the pursuers just as Claude Lebel pursues the Jackal. The Conspiracy asset works his way to the target, using Disguises and tricks to throw off pursuit, hiding in plain sight. Then comes that fatal moment - it might be outside Number 10, or in Westminster Abbey - when the agents either snuff out the threat, or watch helplessly as the Conspiracy claims another victory.