Depends on the bike.

If it has horizontal dropouts, it should be fairly easy to convert back and forth.

If it has vertical dropouts, it's probably not worth it.

FWIW I don't think fixed gear is ideal for NYC, unless you really think you can impress the ladies with your track-standing skills and [whatever the hipsters are wearing nowadays to replace trucker caps]. Geared bike should be fine and gives you a bit more versatility.