I am not completely sure about the US but I will take a stab at it anyway.  If you sell your Bitcoin back into fiat, spend it on altcoin, or spend it on anything then you would pay a capital gains tax on it or take a loss depending on if you purchase something with Bitcoin at a lower Bitcoin value than when you bought it.  My suggestion would be that if you are a long term holder but want to spend bitcoin to buy Alts is that you stash your long term bitcoin holding away in a wallet to grow and just buy bitcoin on the exchange and immediately turn around and buy the alt with it not mixing it with your long term stash or you will have to pay tax on it by way of FIFO.  Sure, with the volatility of bitcoin you may have a small loss or gain by the time you buy the alt but you'll just have to try to keep track of it.  I am by no means a tax expert but I plan to consult one down the road when I want to start spending my accumulated bitcoin.  My advice is to be honest and not cheat the tax man in the US even though I don't know anyone who likes to pay taxes.