> In _The Firebird Book_ it states that changes to the column type are not
> applied until the data is requested, and that a backup/restore should be
> considered in order to reduce performance problems.
>
> Would it be valid to select all items from the modified columns in order
> to force the type conversion?

No. When you just select data, the conversion is performed
on the fly, nothing is changed inside database.
To change data permanently, you have to create new record version,
e.g. by dummy update
Update Tab Set id=id;

Ivan
http://www.volny.cz/iprenosil/interbase/