I'm just curious if Bitcoin has the capability to change and evolve in order to counter any vulnerabilities or other unforeseen issues that may arise, or if everything about the nature Bitcoin was set in stone at the time of original implementation and that it would be nearly impossible to go back and make any fundamental changes to how Bitcoin works.

One of my biggest concerns about Bitcoin is that there are superior (presumably) virtual currencies popping up all the time that purport that they overcome certain weaknesses of their predecessors.  It appears that the only reason why Bitcoin continues to be the incumbent is because of its first-mover advantage and the apparent critical mass that's working in its favor.

But what would happen if a sufficiently superior currency is introduced - one whose superiority overcomes Bitcoin's critical mass advantage?  Will that be the beginning of the end for Bitcoin, or was Bitcoin originally designed in such a way that it has the flexibility to actually evolve and perhaps incorporate into itself the any significant improvements brought upon by some new currency?

PS: I understand this is somewhat similar to another thread here entitled "Cryptographical Arms Race" but I think it's sufficiently different to deserve its own thread.