"SAVE devices cannot be converted to other Symmetrix devices."  This is coming from the SE 7.4 Array Controls Guide.  If you need the space you'll have to delete the SAVE dev and use the space to create a DATA DEV.
I have some devs that are in a snap pool that I want to move to a thin pool. I can remove the devs but that still show up as a save device. Can a dev be converted or do I have to destroy it? 
Converting a save dev to data dev