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....??
At the moment, I would have chosen option 3, because the market is very unstable, and in the near future there should be a serious correction of what I have no doubt about.