"Military exemption should be reviewed for abolition starting from this competition"... Ahn Cheol-soo says "It only requires the President's decision"

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Ahn Cheol-soo seems to be saying that it can be applied retroactively because it is a Presidential Decree.
But even a Presidential Decree is a legal norm under the rule of law.
Shouldn't the prohibition of retroactive effect and the principle of protection of legitimate expectations apply as they are?