One of the earliest bitcoin bulls has a message for investors in the cryptocurrency market who have already made a killing: Lock in some of those profits.

"If you are sitting on 20x, 50x, 100x your money on a crypto investment, it would not be a mistake to sell 10%, 20% or even 30% of your position," Fred Wilson, a venture capitalist and co-founder at Union Square Ventures, said in a blog post on Sunday.

"Selling 25% of your position on an investment that is up 50x is booking a 12.5x on the entire investment, while allowing you to keep 75% of it going," Wilson said.

Digital currencies have skyrocketed in value over the past year. In that time period, bitcoin — the biggest cryptocurrency by market cap — is up more than 1,500 percent. Ripple, meanwhile, ripped 35,000 percent last year alone.

But it hasn't been a smooth ride higher. Cryptocurrencies are prone to wild price swings in very short periods of time. On Monday, bitcoin and ripple fell 9.2 percent and 11.7 percent, respectively.

Volatility is also expected to persist in the digital currency market throughout 2018. Ari Paul, chief investment officer of cryptocurrency investment firm BlockTower Capital, predicts that bitcoin will trade at both $4,000 and $30,000 at some point in 2018. Bitcoin would have to drop nearly 74 percent from its Monday levels to reach $4,000. It would also have to rise about 100 percent to hit $30,000.

Chatter about whether bitcoin is a bubble or not also remains. Last week, Jeremy Grantham of investment management firm GMO told his clients the digital currency is a bubble that may crash soon. Grantham is known for predicting the market meltdowns of 2000 and 2007.

"I know that many crypto holders think that selling anything is a mistake. And it might be. Or it might not be. You just don't know," Wilson said in his post.

Click here to read the full blog post.

— CNBC's Evelyn Cheng contributed to this report.