I think just about every liberal bourgeois democracy has a way of changing the constitution. BTW if we ever got to the point of actually have support for abolishing the Senate (and you can't rule this out - an actual revolution might choose to carry out changes via constitutional means as a way of maximizing public support) there may be a way around that provision. Granted you can't abolish the Senate without the consent of every state - but you could do to it what the UK did to the House of Lords - greatly weaken it. Transfer most of it's powers to the House of Representatives (or whatever succesor body you replace the lower house with), give it some weak but real powers - such an the ability to delay implementation of legislation or force a revote in the main legislature. You would need a sympathetic Supreme Court or enough public pressure to force them to be sympathetic. But I can't see the issue coming up except in the context of a pretty strong social transformation in any case - in which case the weakening of the Senate would be one on a long list of radical social changes, and by no means the most radical of those changes.