You do not need to move the data off the tablespace, however, you cannot lower the container sizes below the high water mark. If there are now fewer rows than previously in the table, the high water mark can be lowered using db2dart (but you may need to reorg first).
If you have more than one container in the tablespace, you need to resize them together in the same SQL statement, otherwise as soon as you resize one container, DB2 will try to rebalance the containers.
You can see the size (including high water mark) with:
connect to sample
db2 list tablespaces show detail
For each tablespace number, you can see the container sizes with:
db2 list tablespace containers for 0 [this is the tablespace number from above command).
M. A. Feldman
IBM Certified DBA on DB2 for Linux, UNIX, and Windows
IBM Certified DBA on DB2 for z/OS and OS/390