Good question! I don't have a specific answer for that other than there are millions and millions of apps and databases running in the world that would all have to be changed at once. So, it's not something that could be "slipped" in unannounced or without the agreement of millions of wallet holders, miners, and etc. In the interest of full disclose, I don't even know who the parties were who created this particular system in the first place or who even does know. ;)
I wonder what the IRS would do if someone tried to pay taxes with a bitcoin.
Oh it could be done ever so easily. See the bitcoin guys could go well since that first 21 million bitcoin printing operation is chugging along we are going to set up another one. Now this one will be separate algorithm but they are both called bitcoins and are exactly the same.
I mean there are no rules to say they can't do such, after all they have the same power as the FED does over creating dollars. There is no Government Agency regulating this currency.