[Mastador1]

Don’t you kinda give up a few rights when you enlist, agree to follow certain military codes and rules?

[Jet Jaguar]

You do.

This guy was already serving.

They changed the rules without a grandfather clause.

[OneWingedShark]

Don’t you kinda give up a few rights when you enlist, agree to follow certain military codes and rules?

May I direct your attention to two pieces of information in the article?

• Staff Sgt. Adam C. Thorogood
• under regulations revised in 2011

You don't get to staff sergeant in three years, so the terms he agreed to were different.
The constitution places this regulation on Congress:

No Bill of Attainder or ex post facto Law shall be passed.

While the Supreme Court did carve out an exception for civil law, the possibility of retroactive rule-changes impacting punishments was their main argument — so if these revisions are applied to the staff sergeant then he has a pretty valid case. (Incidentally this shows why things likke military regulations, and 'normal' civil law should, at that high a level, be relatively stable.)