Selling the bull and buying the bear is the perfect way of doing trade. When we talk of holding, different users have got different views. Some decide to hold for a reason, some just buy and forget it. Some keeps it idle, for those people it is good to observe the market and sell when the market is bullish and buy back when the same turns bearish.
Also what OP have suggested is also good, when we find one of the cryptocurrency under hold is in the bullish move go sell it and buy some other potential coin that is in the bear market. This practice is good, because we'll get understanding about different projects than sticking to limited projects. However we need to be careful while swapping from one to the other, because we need to go through different parameters before investing. Later it shouldn't be a tragedy investing on a hype coin and suffering to sell.
Just buy and forget it if we look at the current market we can stress ourselves because it doesn't move like what we want so long term mentality is needed here. we can look for new hope from a week ago and now that hope is gone but it's normal I think because right now the world is in a big mess condition. Dont forget to buy using your cold money so you won't get a headache because of the volatility.