Is just after your PhD a good time to switch your research area?
This decision requires a fair amount of courage and planning. In short, yes, it is very much possible to switch your research area. Whether or not such a switch is successful depends on how well you plan it, and on the supportive nature of your future employers. You should be upfront with your postdoc advisor about your intention to switch. Because of the learning curve associated with the new area, expect a reasonable amount time (6 months or 1 year) before you can hope to produce research output in the new area. They should know this in case they are hiring you with the intention of fulfilling some funding obligations/deliverables in short time frames. My suggestion is to see if you could try to find a link betwee your current and intended area. If so, you could use this link as a bridge to move towards your new area, instead of all at once (which can be very overwhelming).