You said the transaction can be replaced - it can only be replaced if it hasn't gotten any confirmations, right?
Yes, and transactions can be double spent even if they are not marked as RBF.
Read this:
https://bitcoin.org/en/you-need-to-know#instant about confirmation and how many of it is required.
I am asking, because I sent a btc transfer to my exchanger after I received this transaction, and even though the incoming transaction has long been confirmed, my transfer to the exchanger is still pending with 0 confirmations for about 13 hours now.
If you see your transaction as confirmed on blockexplorers (or in your wallet if you run a full node, or SPV wallet with enough confirmation) then that is their service problem that has not yet updated. Possibly their wallet is not synced with the network.