Of course you don't have to. My point was that the further away you go from the way the game is described in the books, the more stuff you have to figure out by yourself. At some point the game can cease to be Dresden Files and become another FATE variant. So if I think about DFRPG in other time periods, I focus first on settings that produce something that is as close to DFRPG as possible except for the time period. What is it after all that makes a game Dresden Files? The mechanics? The genre? The setting?
The Title.
Really, you can play any gerne-setting (Dresden Files, in this case) using practically any game system. Some are going to be a lot more work than others to convert appropriately, and, as you suggest, some will simply suit the setting-genre better than others.
But, my plan is to play Dresden Files in a different time... the same genre-setting slightly tweaked for the period. So, I think it'd be best to use DFRPG similarly slightly tweaked. Converting DFRPG to a different time period will be less work, I think, than converting a different game to DFRPG.
Anyway, what's got to change?
- Skills & Stunts Some skills, stunts and trappings won't fit the time period (mostly, anything related to cars and computers, in DFRPG). A few others may need to be added (notably sailing and ship-related stuff, if I'm going with the Pirate-based game).
- Hexing Complex electronics don't exist, and so another target for hexing will need to be identified... Gunpowder and chemical processes? Early clockworks and other machinery? Compasses? Or perhaps it's the classic signs of of the power of witchcraft, like nearby fires burning odd colors, animals spooking in the presence of wizards, milk curdling, etc...
- Other Incidental Changes There's going to be some other relatively minor changes that are simply based on the change in scenery... In a pirate game, for example, the list of gear will need to be changed to include muskets, cannons and cutlasses, but it doesn't really affect any mechanics, it is a rather obvious change, and can be handled as you play the game.
Am I missing anything? It seems that, mechanics-wise at least, there aren't a lot of changes to be made. Most changes will lie in the historical background and plot.