Old legacy or not, this is still the CURRENT Constitution that used by R.O.C or Taiwan. And it is this Constitution gives the current Tsai government the power to rule the Taiwan island.
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Like it or not, constitutionally, both Taiwan and Mainland China belongs to China. No matter the China is PRC or ROC.
People in Taiwan has the constitutional right to change the current constitution. No matter it is the article about the name of ROC, or the article about territory. The ROC Constitution gives them the right to do so, just like the ROC Constitution mandates that both Taiwan and Mainland China belongs to China.
Of course, once such change happens, there would be consequence, very serious consequence. I hope they won't be that mad on this.