Change one Visio shape to another without deleting.

Hi!

I have just been given a load of Visio flowcharts and been asked to change their shapes. The problem it could take a long time to replace the current shapes e.g. processes used, with something say 3D like.
Is there a way to change an existing shape e.g. a process to a shape from another selection e.g. a 3D box?  

Note this is important, hence the point value.

Thanks.