when is it best to move plants from seed cells to pots

I would certainly pot them on now. they will have a good root system and would welcome the room and fresh compost.

the thing with plants is generaly it is not natural at all to be moved once theyve rooted.its us humans that do that for them.id say as long as you dont disturb the roots like you shouldnt realy anytime is good .its better in winter when there dorement especialy big plants that need digging up because they dont efectivley know theyve bean moved.theres no air born bugs,germs etc lots of reasens but as your just giving your plant more space for its roots without disturbing it it should be fine.very late in summer/early autumn is possibly the worst as all the said bugs and germs are about and its got to get through a winter without getting a chance to settle down.this is a general rule of thumb for most plants.there is always an exception to general rules though.