I get the analogy to gold vs. gold-backed dollars, but in Bitcoin's case I can't think of any specific changes the Feds could make to a fork that would allow it to both (a) obviate almost everyone's need for Bitcoin and (b) be enough different from Bitcoin for the government to be satisfied with it (and hardcore bitcoiners to be dissatisfied with it).

For example, if the money supply remains un-changeable, this is awesome as the Federal Reserve has effectively been neutralized. If they make a fork that can be inflated, Bitcoin Classic retains its store-of-value properties and will be highly sought after. If they do something in between, we get a little of both. If they remove all anonymity possibilities in their fork, Bitcoin thrives in the black market; if they don't, awesome. If in between, a little of both.

At one end of the spectrum they take just a few of Bitcoin's properties to build a system that is better than current fiat, which would be moderately liberty-promoting, while still leaving Bitcoin's differentiated value proposition very firmly intact. At the other end of the spectrum they adopt almost all of Bitcoin's properties, which would be extremely liberty-promoting, but leaving Bitcoin with little differentiated advantage. Either result is overall very good liberty-wise (though the result where they mimic Bitcoin quite closely may not be very good for Bitcoin investors - but that result is also highly unlikely), and I see no point in between those extremes that would be any worse. So I conclude there is nothing to worry about.