The validity of the employment visa will be cut short if the validity of the passport is not sufficient to cover a full year. If you have renewed the passport before using the visa for Hong Kong, you may re-apply for a new visa but you have to go through the application process again and pay the visa fee for the new visa.
If you use the visa for Hong Kong, then renew a passport, you may then apply for extension of stay to our Extension Section and our colleague will grant you the full period of employment in your new passport.
The choice is up to you and will hinge on whether it is convenient to renew a passport while you are staying in Hong Kong. The application procedures with Immigration Department are not complicated.