Can serial number on laptop be changed if stolen??

hi,

my laptop was stolen a couple of months back.

I saw it in a second hand shop electronics shop window last night! near my flat!

the policeman took my serial number from me, but he said it didn't match the one on the outside of the lap top.

I am 100000% sure it's mine as it has a distinctive crack on the front bit where I dropped it.

Is there anyother way I can prove it's mine??

Can the serial number on the outside be changed??

thanks