How that wasn’t blindingly obvious from the start is a question, but not one he or his supporters actually care about because they couldn’t give a fig about Ukraine. It is entirely instrumental to his personal political advantage in the moment. Being utterly opposed to support for Ukraine was politically advantageous in opposition and supporting Ukraine to the hilt is now politically advantageous in power. That’s all that matters.
Chemical weapons in Syria are an informative parallel. Obama’s commitment to bombing Syria if they used chemical weapons was the worst policy ever from opposition, but actually bombing Syria for using Chemical weapons when they did so as soon as Trump gained power was an obvious necessity.