I have developed a small database using Liferay Service Builder, the target is an Oracle Database. I am wondering if it is possible to change the field type from VARCHAR2(BYTE) to VARCHAR2(CHAR)

Just change the tables.sql before you deploy the plugin.

Liferay will invoke the tables.sql to create the tables only if they do not exist. Any updates to the DDL going forward must be done manually, so you needn't worry about Liferay trying to change it behind your back...