New Drobo Volume After Upgrading Hard Drives

One of my clients has a Drobo 5D. Four of the five bays were filled 4 TB drives. I added a fifth drive (to the 0 slot), an 8TB drive. The Drobo seemed to recognize the drive and used it for additional capacity. I then pulled one of the 4 TB drives and swapped in another 8 TB drive. This time, instead of automatically seeing the extra space and using it, the Drobo asked me to create a new volume.

What do I do at this point so the Drobo adds the additional storage space to the current volume without destroying the volume and the data currently on it?

Sounds like it was initially set up using a 16 TB volume size. When you add capacity that exceeds this, Drobo will ask for another 16 TB volume to be created. This is normal.

You can either allow Drobo to format the second 16 TB volume and use it like any other drive (it will have another drive letter), or (if the firmware is up to date) copy the data off of Drobo, reset it and allow it to create a new 64 TB volume, then copy the data back onto the Drobo. There is no way to expand the existing volume size without wiping the drives.

same problem (firmware and dashboard are up to date)

Same solution.