"Does anyone know who to reclaim this space"
You will have to format the partition which will also wipe out your data in that partition. Use one of the mkfs commands to format the partition, depending on the file system type.
"have a tip for what command to use next time I want to do something similar?"
mount each of the partitions on a directory and then use the cp command to copy the contents of one directory to the other directory.
Be prepared. Create a LifeBoat CD.