At present you may be having plastic earing, if you are going to change with golden or platinum rings then you can do any time or if you are changing it with some imitation jewelry then dip your jewelry in some antiseptic soln. and wear it . You can change it no problem.
You probably could, just make sure the earrings your changing them to are pure silver or gold ones,NOT SILVER PLATED! And wash them in some kind of disinfectant (that Detol stuff works) and clean the pierced hole.
I pierced my ears myself about 5 weeks ago and changed the earrings last week. Mine haven't got infected or anything.
Even after cleaning the piercing and earrings it is still possible to get an infection, if you wanna risk it go ahead :)