You won’t lose them but at present you don’t own EOS tokens. You own an ethereum ERC20 token with a promise to give you the equal number of EOS on the EOS block chain when it goes live. Therefore you need to register an address and link it to your ethereum address that holds the EOS. Shortly after the presale ends, a scan of EOS ERC20 holdings will be taken and relevant Addresses on the EOS blockchain will be credited with the relevant balance.
If you don’t register, you won’t lose your current tokens but you won’t receive tokens on the EOS blockchain and what you hold will be effectively worthless!