1) Setup space in the OS for the new DB
2) Use dsmfmt to create the new DB volumes (dsmfmt -m -db <file> size_in_MB)
3) Define the new DB volumes to TSM (define dbvol <vol>)
4) Extend TSM DB by new DB vol amount (extend db <size_in_MB>)
5) Reduce TSM DB by the old DB amount (reduce db <old_db_size_in_MB>)
6) Remove old DB volumes from TSM (del dbvol <old_db_vol>)
7) Delete old TSM DB volumes from OS.
Then again, you could just create a new volume and add it to the existing DB and just make your DB bigger rather then move it to a bigger volume.
-Aaron