How to change format of External Hard Drive, without losing data on it?

I have an 8 TB External HDD, that is formatted to work on only Mac. It has 2 partitions, each formatted with Mac OS Extended (Case-Sensitive, Journaled, Encrypted), split for 4 TB each. I don't have room on my iMac to take either of contents from either partition onto my HDD, so I can delete the partition, and I don't have another external HDD that's large enough to accept the files. The new computer will be a PC, and I need to keep the backups that are on this external HDD, as there are about 3 or 4 computers backed up on it.

Is there a way to reformat external HDD without losing the data, from my iMac (mid-2010)? Or do I need to find 8 TB worth of storage to put the data onto, in order to erase and reformat the external HDD?