Here is the GP's switching guide:
http://www.gpnotebook.co.uk/simplepa...?ID=1651179592
That matches what I've seen on several local trust docs issued to doctors in their regions.
With some of these meds you can just switch the next day or cross taper under supervision. Where meds work on Serotonin, there is going to be some overlap even if they work a little differently so this is supposed to make the switch less harsh as you are not going all the way to 0% before starting the next one.
Plus it takes a while for a med to be eliminated by the body. For instance, LittleMissAlone's Sertraline would take much longer than those few days to be eliminated so she had the benefit of some of that still in at the time.