Aragorn would have to have changed at some point given that at the beginning of the movie he was a sissy who be his actions could never have become king. I think the reason for his change was that he saw that Boromir believed him to be the rightful air to the throne and that the people of Gondor would follow him. This gave him a boost in his self esteem which made him believe that he could be king, something he seemed to doubt. The taking of the wristguards signifies that he is now ready to be king, and that he would die for Gondor in order to protect it.
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His sword was long his lance was keen
His shining helm afar was seen
The countless stars of heavens field
Were mirrored in his silver shield
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