If you have a spare PC lying around, check out the XPenology forums [1] for the software (specifically, NanoBoot or XPEnoboot) required to create your own Synology clone.
You'll be able to create a new SHR volume with new disks. As suggested in another post, do a file level copy to the new volume. You should then be able to replace all the disks in your actual Synology NAS with your new disks at once, and perform a migration [2] so everything works with your Synology hardware.
(This is possibly a better solution than replacing the disks one by one, since there's the risk of another disk failing while the RAID is being rebuilt.)
If you have a spare PC lying around, check out the XPenology forums [1] for the software (specifically, NanoBoot or XPEnoboot) required to create your own Synology clone.
You'll be able to create a new SHR volume with new disks. As suggested in another post, do a file level copy to the new volume. You should then be able to replace all the disks in your actual Synology NAS with your new disks at once, and perform a migration [2] so everything works with your Synology hardware.
(This is possibly a better solution than replacing the disks one by one, since there's the risk of another disk failing while the RAID is being rebuilt.)
[1] http://xpenology.com/forum/viewforum.php?f=2
[2] https://www.synology.com/en-us/knowledgebase/tutorials/484