I have Synology DS716+II, recently upgraded my HDD to 8TB - all in one storage pool using RAID 1 with data protection. And without paying much attention, a single volume takes up all the space.
I was able to upgrade to DSM 7.2 without any issue, but when it comes to the whole volume encryption, I am out of luck.
I have “no available storage pool for creating a new volume”, and “no drives are available for storage pool creation”. 
Since I cannot shrink an existing volume, I am left with no choice but to back up my data and remove the current volume to start over again, is that right? 