You need to swap keys with yourself. To do this, you need 2 DS's, in which you use the Unova Link system to swap keys. Once you select the Link system, select Keys, and all the keys you have unlocked will automatically be swapped. It's best to have the DS's cartridge points pointing towards each other, otherwise InfraRed won't be reliable.
To switch from Tower to Hollow or vice versa, you need go back into the key system and choose Tower/ Hollow as your city. It'll also allow you to switch from Iceberg to Iron Room (Regice and Registeel) if you've unlocked them.
Note that you cannot switch between Tower & Hollow cities if you last saved your game in your game's current city.