can you extend 7 days visa for 30 days?

Hi there, just got my passport with an entry permit and realise that i just got 7 day one.
It was my mistake and in ambassy they told me that i can't change it.
Is it posible to extend 7 days one for another 30 days?
Or its better to wait till it run out and apply again?
we are in Vietnam atm so maybe we could go to hong kong and apply there again.
Is it posible?
Many thanks
Jarek