Consultancy needed on vmware workstation

I need some help with the following problem. I have created a number of virtual machines with vmware workstation 6.5.? ? I have created them without pre-allocating diskspace.? But now I am? looking for a way to improve performance an I have heard that pre-allocation disksize increases performance. The problem is that converting the virtual machines with vmware workstation does convert the machines to ones with pre-allocated disksize but does not preserve snapshots. I need a way to convert the virtual machines without losing the snapshots. I allready tried defragmenting the virtual machines but the performance gain is not satisfactory.