DB2 Alter column type and index state RBDP vs. "needs a rebuild"
The issue here is that you're reading documentation for Db2 on z/OS, but you're running tests on Db2 for Linux, UNIX &amp; Windows.  

When you perform this particular alter on Db2 for Linux, UNIX and Windows, it will succeed, but the table will be placed immediately into reorg pending state, which you can see in SYSIBMADM.ADMINTABINFO (see the REORG_PENDING column).  INDEXES_REQUIRE_REBUILD will still show as 'N', even though performing a REORG on the table will rebuild the index.