Question about DS1819+ - 338018

I wouldn't bet that the disks will be recognised 1:1 after the conversion. Even if this should be possible according to Synology under "HDD Migration".
With such a migration it is always advisable to back up the data separately on other data carriers so as not to be left with nothing if a migration goes wrong.

The RAID type can be changed afterwards, but not without restrictions.
Furthermore,

RAID6 is generally not recommended because of the double parity etc etc.. Neither for home nor for "business". A RAID5 with hotspare that takes over if a disk fails is more recommendable or Synology Hybrid RAID. (With >= 4 disks, the fault tolerance here is also 2 disks as with RAID6).

My suggestion would be
- buy a new DS1819
- Install 4 x 3TB new disks
- Set up Synology Hybrid Raid on it
- migrate via Synology Backup & Restore over the network ... the original files will not be affected
- Verify that everything has been successfully restored to the new DS.
- Remove the old disks in the old DS
- Install them in the new DS
- Expand the storage pool / RAID type with new drives via the storage manager of the new DS.
- Disposal of the old DS

A lot of information in German is available on the Synology pages at www.synology.com/de-de/knowledgebase/DSM/help/DSM/StorageMan....