I want to repot a pieris

Id say when it is dormant say mid Autumn ie late September mulch the roots with plenty of peat or peat sustitute as they love acidic soil & it should flower again in spring.

often if you are going from pot to pot it doesnt really matter as long as the roots dont get disturbed too much. if the plant is pot bound [true for the acer in the other question] gently tease the roots out and then after repotting water well for at least a fortnight. dont rely on rain being enough as the leaves will shelter the pot. so despite rain you must water it.