TRAP!
A bespectacled functionary sits at a desk (HP: 8, AC: 2, MV: Won't, ML: 11 Att: Special). He is the guardian of a terrible machine that can draw outside the implied PC inside.
He will calmly explain that the machine only goes one way, no refunds. Also, it won't turn out the way you want. Also, you are prohibited from using it at all.
Although he won't directly stop the party as they barge past, if they do so,
(or attempt to stab him to death or otherwise interfere, successful or not as he is a slippery little fella - and even if killed dead he will do the same, although afterwards expire theatrically)
he will stand up, greatly aroused, and make GRAND PROCLOMATIONS in a surprisingly booming voice, advancing:
1 Disquisitivism
Leave no stone unturned! As a CONFUSION spell as party investigates.
2 Excessiveism
Redundancy is the new Theurgy! As a bastard child of HALLUCINATORY TERRAIN and MIRROR IMAGE. Instead of a machine, there are copies on copies of the party. There are so many of them they can scarcely remember who is the original.
3 Sensory Conflictism
As the planet moves, so should your inner ear. Motion sickness all around. As HOLD PERSON.
4 Symetryicsim
Kissing cousin to Dichotomy Paradoxism - That which is in locomotion must arrive at the half-way stage before it arrives at the goal - even further, as you step forward, the scenery steps back, cha cha. As WALL OF ICE.
5 Capcaisinism
Spicy! As a 1/2 strength FIREBALL cast by a level 6 Magic User.
6 Rapacitism
Drink deep and greedily from the cup until, like a squeezed tic, you pop. It ain't hedonism, it's an physio-electro-mechanical imperative. Clown Corp doesn't enjoy drinking the brains out of your skull per se, but drink it does, and heartily.
Effect is similar to a chain of HASTE spells, but really, it's its own thing. Party is moves like highly caffeinated toddlers - so fast they don't have a lot of control over themselves. Anything they want to do, roll 3d6: if any of them are a 6, they succeed (while the other 3 segments they are doing any one of 100 other things), but a particularly complex action (like casting a spell) mostly likely needs AT LEAST 2 6s.
Problem is, on a 1, they can't help but interfere with their fellows because "MINE", undoing the next 6 following, unless the second gives them a sharp blow (1d4 damage, automatically hits) to keep them away.
The machine unfolds thusly, doing its magic on the first person through the apparatchik's cordon of conceptual complexity:
a Thief finds Reliijon and becomes fire-breathing Cleric
a Cleric gets corrupted by forbidden Books and becomes demon-haunted Magic User
a Magic User gets his brains scrambled by an unhealthy diet of sugar and eldritch electricity (and perhaps acquires a resultant Alcohol problem) and becomes quarrelsome Fighter
a Fighter gets his A** handed to him and become jumpy, jumpy Thief
(Please note that subclasses go the OTHER direction FOR THE MOST PART)
a Paladin's faith goes up in a puff of hubris and Disappointment, becomes flim flam artist Illusionist.
a Ranger's trip TOO far into the wilderness changes the whole deal for him - there is a woods inside woods inside woods like a tree matryoshka. Only in the well-ordered grove of a monastery can he calm his nerves through study in combat and Monk mediation and untangle the awful ramble.
an Illusionist's phantasmagorias over time go all Jungian-granola archetypes, becomes spooky Druid
a Druid's sacred glade is burned down by Industrialist, becomes revenge-obsessed Assassin
a Monk's side loses in factional dispute following the death of the Master, becomes sad circus Acrobat
(or if you must, go ahead and call him a circus Bard - I choose corde lisse over chords lute, but realize this is not a universal or even popular opinion)
an Assassin's botched attempt to kill a Demigod results in a shift in targets and in times - they the determined holdout, an Onada Hiroo situation, waiting for a second shot at the diety. Alas, the span of a Demigod is but slightly less fleeting than a man's. The world has moved on, and it has completed the work of this grizzled now Ranger, whose assignment cannot now be completed.
an Acrobat's fall from an obscene height pops him into a sword-dimension where he is handed some sort of armament of justice by an extraterrestrial force. It ain't a Holy Avenger per se, but it makes him a swashbuckling Paladin out of him anyway.
(or if you gotta do bards, make it an EPIC carousing session - wine, wo(men), and song in any order)
(and although the use of demi humans is strongly discouraged in this particular setting, in any other that such a machine were to be constructed, Elves become Orcs (war and industry exerted on a sylvan temperament), Orcs become Dwarves (hearth, mound, enclosure, and roof remold the battle monster to a boaster of an dim-lit antique past), Dwarves become Halflings (give up the gold for butter, the dirge songs for dancing), Halflings become elves (by travel along the Straight Way perhaps, even over generations or in spaceships they stretch out, or at least they seemed shorter from here - see length contraction).
(Gnomes are Gnomes and there's nothing to be done about it)
(all XP remains the same, so levels might fluctuate depending on the transformation).