Oracle: Decreasing PCTFREE on large table
My question was answered on another message board. The answer is to use DBMS_REPAIR.SEGMENT_FIX_STATUS after the change in PCTFREE.
On my platform (sparc64) it's quite buggy, though, with lots of ORA-600 when repairing individual blocks. But repairing all blocks seems to work better for some reason.