By claiming to own mainland China (and Mongolia; CKS was greedy), the ROC is playing by the one China rule.
Beijing's take: "Of course there's only one China, ranging from Xinjiang in the west to Taiwan in the east, and the capital of China is Beijing."
Taipei's take: "Of course there's only one China, ranging from Sinkiang in the west to Taiwan in the east, and the capital of China is Taipei."
If the ROC constitution is amended to relinquish that claim, it is a de facto move toward independence from a greater Chinese framework. Depending on interpretation, this could violate Article 2 of the Anti-Secession Law:
Article 8 continues:
So, one argument goes, if Taiwan makes any changes to its constitution which emphasize its separateness from China, Beijing is not just allowed to but legally required to start shooting the place up. Of course, in reality, it depends on just how much latitude Beijing is willing to give Taiwan, though we have no way of knowing for sure how it would react.