Mobile phones sim change?

im aboutto buy a new phone with virgin

its a pre paid phone and im going to be using my old sim card which is on virgin

but i want to change it to optus but i can't find the phone that i want to get with optus so im buying it through virgin

once i have bought it am i able to get an optus sim card so i can change from virgin to optus?

if so does it cost or anything and how would i do it?Mobile phones sim change?
Yes but you have to pay an unlocking fee to virgin

which is about 60 to 80 dollars unless you wait 6 months till it's unlocked