Will the 1 and 2 euro cent coins lose their value now that cash payments have to be rounded to 5 cents?
No. The coins will not be taken out of circulation. You can still use them to pay. And the merchant can still use them to give money back.
If you still want to exchange euro coins at the National Bank, you can do so by presenting them in bulk (not packed) at the counters of the National Bank, Boulevard de Berlaimont 3 in Brussels. There is no need to sort them. Every Belgian of age can exchange up to 5 kg of euro coins once a month.
More information can be found on the webpage "Exchanging euro coins".