Your altcoin goes up 50%. Options:
1. Hold. You think your alt will beat BTC returns.
2. Sell initial investment to BTC, leave rest in alt. You think BTC will beat your alt (or you are unsure).
3. Sell initial investment to USD, rest in alt. You think BTC and alt will go down....??
For what is happening now into the flow of the value in the altcoins in the exchange site was very unstable actually. Only few of them are can be considered as real stable in price value which is Ethereum. So I'm not sure if there is 50% goes up in the altcoin while bitcoin was being dumped now so far.