How to change column datatype when it has dependencies?

It is not directly possible with Firebird. If you try to do it with ALTER TABLE command, you would get an error like this:

This operation is not defined for system tables.
Error 335544351, unsuccessful metadata update
Column EMP_NO from table EMPLOYEE is referenced in SET_EMP_NO


Some commercial tools use hacks on system tables make it possible, however, make sure your database can be backed up and restored after those. Also, make sure you can extract metadata for entire database, and recreate it afterwards. The reason is that system table modifications are tricky, and one small error can show up months later when you completely forget about it. Even if it works, problems can occur, as your stored procedures and triggers are compiled into binary BLR and they might seem to work, until you try to alter them once and use the original SQL source which is only stored as text in metadata tables. Even if those changes are well tested on databases, newer versions of Firebird might change the structure of system tables and some new rules might go un-noticed by tool makers.

The safe way to do it is to remove all dependencies: drop dependent foreign keys, check constraints and views, alter procedures and triggers to have empty bodies and then change the column. When done, alter the column and recreate all the dependencies. This will ensure that you catch any incompatibilities with the change you made to column's database, as the creation of those objects would probably fail in such case.

This might seem like too much work, so use tools to help you. For example, FlameRobin has such feature called Generate Rebuild Script. It is available at the bottom of properties page for each table. It also generates such scripts automatically when you select ALTER VIEW command.