The document below has some information pertaining to data in-place upgrades to D@RE. There is a potential issue if your block code is less than 5.33…081 or your enabler is less than 01.01.5.004, so make sure you are at the latest revs before you do this. How long it takes depends on a lot of factors, but I would measure the process in days not hours.
You also have to disable FAST Cache before you start the process, so you will take a potential performance hit. You can re-enable FAST Cache after the process starts, but it will take a while to warm-up, figure hours for that.