It is widely noted that Emma Bull pioneered urban fantasy with her 1987 book War for the Oaks, splicing the fantastic with modern mundane grit. Since then, writers, artists and game designers have been churning out magical tales set in bustling metropolises, such as the ever-popular Neverwhere by Neil Gaiman and C.J. Carella's brooding roleplaying game, Witchcraft. The genre has been shifted right out of the underground in recent years by Buffy the Vampire Slayer and Guillermo del Toro's adaptation of Mike Mignola's Hellboy, and is capturing more imaginations than ever before.
So it would only be natural to cast our beloved T&T fantasy exploits into a time-warp, shooting it into the here and now. Classes can stay as they are: magic-user and warrior (if you're using 5th edition rules) but creating sub-categories in keeping with the modern time frame would be a good idea. If you're a warrior then you could become a fed, a soldier, a cop etc. A magic-user will still be using magic, but warlock, witch or, indeed, wizard, would suit the setting. Of course, you don't have to use subcategories; you can just as well have a warrior and call her a warrior.
Swords, bows, axes and the like all have their place in urban fantasy so they can stay. Guns are obviously more prevalent and advanced so use the same rules for Gunnes in the rulebook but replace flintlock, handcannon, musket etc. with modern variations like 9mm, shotgun and M16. Automatic weapons fire more so use more dice. Similarly, armour is transformed from plate, mail and leather into varying grades of flak jacket, army helmet and riot shield.
For campaign inspiration, check out The Dresden Files by Jim Butcher, Kim Harrison's The Hollows, and Holly Black's Modern Faerie Tales. If you're familiar with Carella's Witchcraft then by all means borrow elements from that universe.
If you do decide to have a go at Trollpunk, let me know how it goes.