Fastest option is to edit 103.conf and add (and adapt) the line you find in 102.conf that are related to that disk (so 103 VM will point to 102 disk).
The problem is that if you then delete VM 102 it will delete the disk too...
If "DATA" is a file system, you could simply move and rename the disk under the directory of VM103 (i.e. cat /etc/pve/storage.cfg, have a look at path for DATA, and under that path/images you have 102 and 103 dir. Move the "disk file" from one dir to the other and add the corresponding line to 103.conf).
If DATA is block device, you have to be very careful and use i.e. dd to block-copy things around (very dangerous command if you don't know what are doing, I'm not able to help here).