Ceramic capacitors often have a very wide tolerance and their capacitance varies with voltage. A ceramic capacitor would typically be larger than a tantalum capacitor of the same value, but would have a lower ESR. So in most applications where a tantalum capacitor is being used for decoupling, you should be able to substitute a ceramic capacitor, if it will fit in the space. OTOH if the tantalum capacitor is being used for coupling audio or for timing, then replacing it with a ceramic capacitor would not be a good idea. I replaced a bipolar tantalum capacitor by a metallized film capacitor once, but typically there may not be enough space to do this, since metallized film capacitors are normally quite a lot larger than ceramics or tantalum caps of the same value.