If you want to use some of the more advanced packages and features that the DS920+ can provide then you will have to start from a new storage pool and volume using BTRFS file system with maximum of 108TB volumes. This will then not have any vestige of the EXT4 file system and 16TB maximum volume size that the DS216j's architecture will have imposed.
If you have a full Hyper Backup of the DS216j then you can use that to restore to the DS920+. Part of this will cause packages to be installed as their configurations are restored. There will be some DSM settings and packages that are not fully restored (less in DSM 7 than DSM 6) so you could run side-by-side DSM browser windows to check for anything that's missing.
Others will suggest to remotely mount DS216j shares on the DS920+ and do folder copies / sync or some such. I've not done this but that's not to say these aren't valid approaches too.
Pretty much covers it. Don't use Migration Assistant because it will force you to have the same pool and volume file system (ie. EXT4) on the new NAS. Which somewhat defeats the objective.
How much you want the DSM and package configurations to be the same on the new NAS will determine if you want to restore them or build up these from new. If it's just the data you care about and not packages/DSM then you can move/copy using remote folder mounts.
If you do use Hyper Backup then that works well to restore the DS216j to DS920+. I did a similar thing:
DS215j had EXT4 and 16TB limit but did a disk migration to a DS218+ (it did work even though the Synology KB says otherwise). This worked but I cannot remember exactly what I did but the data was available: maybe a DSM / package config reinstall from HB vault which filled in the holes that may have been missing.
But migrating from this DS218+ 'legacy' setup to DS1520+ was achieved by initial setup on new DSM/pools/volumes on the DS1520+ using BTRFS and getting rid of the 16TB limit.
Next I restored the DS218+'s full HB task data and configs to the DS1520+.
Finally side-by-side DSM windows to check the missing bits.
Detailed here, and know that DSM 7 backups up more DSM/package configuration info than DSM 6.
Something became apparent after 18 hours of doing a full Hyper Backup 'clone' and rebuild (ext4 to btrfs, and remove an old limit of 16TB volumes) was that not everything is backed up. In the end I powered down the NAS and switched back in the 4TB SHR-1 disks as I've a new NAS on order and I was...
I'm lazy and don't want to manually setup a new NAS after getting the old one just how I liked it
The only issue with @Telos's approach would be that any packages that are installed on the original volume (from the DS216j) will be lost. Until you have two or more volumes in DSM then Package Center does not display the setting to select where packages get installed (...because there is only one volume). But when you have two or more volumes you can select which volume. The pain is that there is no builtin method to move installed packages and their data from one volume to another. Google exists to provide manual ways via SSH.
The result would be that, without doing the manual SSH thing, you could blow away your old volume thinking everything is migrated and then find the packages are gone. Obvs. you can still restore just packages from a HB vault if you have got one.