Is it considered a sin to replace competent non-Muslim workers, whose conversion to Islam is not anticipated, with competent Muslim workers?

If the contract between you and them has expired, then there is no blame upon you for replacing them with other Muslims; in fact, that is better. However, before the contract expires, it is not permissible for you to do so except with their consent to terminate the contract, due to the generality of Allah's Almighty saying: "O you who have believed, fulfill [your] contracts."