Resizing and changing partitions on an amiga to my knowledge will always nuke the data of the partitions. I'd be happy to be disproven though 
You can do it but it's hazardous - ReOrg so the data is in the beginning of the partition; split the partition and quick format the new ones. Disksalv/undelete the new first partition and hopefully get the data back. I never tried, just bought new hard drives...
