If we're talking about the Budapest Memorandum, cirumstances possibly changing are actually part of the treaty. The memorandum conditions the use of force again

If we're talking about the Budapest Memorandum, cirumstances possibly changing are actually part of the treaty. The memorandum conditions the use of force against Ukraine, and makes provisions for what would happen if a party to the treaty or one of its allies attacks another party to the treaty (second and fifth point, one is about conventional response, the other about nuclear response): > [The parties shall] refrain from the threat or use of force against the territorial integrity or political independence of the signatories to the memorandum, and undertake that none of their weapons will ever be used against these countries, except in cases of self-defense or otherwise in accordance with the Charter of the United Nations. If memory serves right, Ukraine didn't invade Russia, and Russia is not under a UN mandate.