milosevic - 6:13 am on May 27, 2012 (gmt 0)

There's no official sandbox period, but if your domain has heaps of links to it that say for example "lawnmower repair" it's likely to be considered relevant to lawnmower repair. If your domain has few backlinks you can consider it more like a newly registered domain.

I would still be wary about changing content completely overnight though and would either take down the current content some time in advance or ideally do that and put up some informational content related to what you are going to be targeting.