1. Do a trick that involves setting the deck on fire. Obviously, after that you'll have to switch! 😆
2. When ready to swap, shuffle if needed (e.g. in case the trick ended with a particular pattern visible to the spectators), ask a spectator to take one of two removable stickers of different colors and seal the deck for you exactly on the old seal (or they choose the color, you apply the sticker for a better match with the next deck, then grab a Sharpie and optionally ask them whether you should draw something on it, a circle or square). Drop it into a large paper cup that has a waterproof card holder with only the top end open and a flap of velcro ready to pop over it. In the bottom the cup already has slush powder in it. Pour in water using a measuring cup with a triangular spout for control. Obviously, pour into the outer area. Tell them you'll make the water disappear and the deck will teleport into your pocket (or magician's box). Do a very quick coin trick if you like, some extended abracadabra, look into the cup, look pleased, invert the cup to prove both have vanished, set it aside, inverted, reach into your pocket or box and pull out 'the exact same deck', having prepared four different tuck boxes, each in a different pocket, sealed with the obvious combinations of sticker color and shape drawn of a special, unique kind of which you don't mind collecting five decks, or just four basic decks. Eco's only $4+ at Penguin, or you can buy another type in bulk. Having it be an unusual kind may strengthen the illusion of it being the same deck.
You can have the contents all be the same deck setup so you control which deck you switch to, or have a different one in each pocket e.g. Svengali, stripper, invis, marked, whatever you're have routines to work with, or four in your magician's box, and you just do a routine with the one that you're made to select, since you know routines for all four). If you plan to later swap again, just peel the sticker off when you produce the replacement deck the first time and toss it so that you can swap more easily the next time (or put the sticker on a blank card, sign below it, and give it as a souvenir). The second time use a different method like David's "Put deck in pocket, hesitate, and say, oh do you want to see another?" or any other you find in the above sources. Having removed the sticker means you don't have to have four copies of each for that next swap. Variant 2: While they put the sticker on, you put away the sticker sheet (for instance, folding the sheet, surreptitiously removing the same sticker color from the back side, which folding can facilitate with some sticker types, moving the hand with the sticker and sheet into your box (having omitted the Sharpie bit earlier to simplify) and put that color seal on the next deck you want to use and break it with a thumbnail. Then later do the "Put deck in box and say, oh do you want to see another?" bit so you can swap yet again.