can we move our bitcoin before the new fork happen? and after?
you can move your bitcoin anytime you want. bitcoin network has been working and will continue to work at all times.
how it work exactly do we risk to lose our coins because of replay attack?
you do NOT lose your bitcoin if you move your bitcoin.
replay attack means the transaction you make on bitcoin network will also be valid on the other (fork) chain too. so it can be mined there too.
imagine you are making a payment to 1BtcAddress... you obviously don't control the private key to 1BtcAddress and you are making a bitcoin payment. that goes one fine without any issues. but the same payment will happen on the forked chain and you also pay bitcoin_forked to 1BtcAddress... on the forked chain. but since you don't control the key to that address, your bitcoin_forked coins are practically lost.
remember this works the other way around too. meaning if you claim your bitcoin_forked coins, the transaction you make on the forked chain is also valid on bitcoin network so you get the bitcoin_forked coins and lose bitcoin!
that is why these chains have to implement a strong two way replay protection or if they don't you have to first split your coins then make a transaction.