Security of a cryptosystem where the encryption is swapped with decryption For any given one-key cipher $(M,C,K,E_k,D_k)$, we may modify it into $(M,C,K,D_k,E_k)$. In other words, we swap the encryption function with the decryption in the modified cipher.

Are the two ciphers have the same level of security with respect to known- plaintext attacks?
If we have a plain text attack which takes n pairs of known plain text known cipher text and with expected computational effort T extracts the key K then for the inverted cipher the same attack would work with the same properties would work by swapping the order of the pairs.

More formally if we have an atack function a(c,p)= k where c is a cipher text and p is a plain text, we can trivially construct a'(x,y)=a(y,x) which will work on the swapped cipher. You can extend this to family if known plain text attacks, multiple pairs required, computaitional cost, probability of succsss, weak keys etc. As long as the input is pairs of cipher text and plain text you can swap the pair order and get the same attack propeties on the swapped cipher.