The differences between tantalum and aluminium is that tantalums have a lower ESR and better high frequency response. For that reason they are normally used in place of aluminium for critical applications, such as switching regulators. So, using tantalum in place of aluminium is usually not a problem - the other way round could be. In fact, if it is for the output of a switching regulator I am surprised it isn't tantalum anyway (or maybe ceramic). The worse high frequency response of aluminium tends to mean more high frequency ripple than if you use tantalum.
Keith.