Is it possible to change a PK datatype from SMALLINT to INTEGER?

I've though I'd removed all the dependants, but get an error
messages like...

ALTER TABLE CATEGORY3 drop constraint INTEG_101;
// $$ ******** ERROR ************* $$
//FAILED - unsuccessful metadata update
//ERASE RDB$RELATION_CONSTRAINTS failed
//action cancelled by trigger (1) to preserve data integrity
//Cannot delete PRIMARY KEY being used in FOREIGN KEY
definition.
//
commit;

SET term ^;

ALTER TABLE CATEGORY3
ALTER COLUMN CAT3NO TYPE INTEGER^
// $$ ******** ERROR ************* $$
//FAILED - unsuccessful metadata update
//MODIFY RDB$RELATION_FIELDS failed
//action cancelled by trigger (1) to preserve data integrity
//Cannot update index segment used by an Integrity Constraint

Using FB1

Thanks,
Steve