Iphone 3g (unlocking and version updatation)?

I own apple iphone 3g currently running version 4.1 (8B117).It is locked on o2 ireland so whenever I connect it to the itunes, itunes ask if I want to update it to the newer version (for example 4.2).

If I update it then it will get locked and since I don't have the original sim that came with it, I won't be able to activate it when I hook it up with itunes again. Is there a chance to get it activated when I don't have the original sim that came with it?

Another query is, since it is locked on o2 network in Ireland, can I get it unlocked like factory unlock from somewhere so that this problem doesn't arise again anytime?

Please share your knowledge.

Http://m.youtube.com/#/watch?v=IOiVVuUoP_Q&desktop_uri=%2Fwatch%3Fv%3DIOiVVuUoP_Q&gl=DE Try what i have found http://community.o2online.ie/t5/Apple/Unlock-code-iPhone-3GS/td-p/137357