First you should measure the inner width of the rim. As you are mounting 32 mm tires, the inner with will be (typically) around 16 and 19 mm. If the width is around 16 mm you can mount 28 mm tires without problem and probably 25 mm tires. If it is around 19 it will be difficult to fit 25 or 28 mm tires but the latter could fit, depending on the shape of the rim.

NOTE: I am talking about rims with with inner lips to hold the tire.