Before Bitcoin Cash, you had 1 Bitcoin. After the fork, you had 1 Bitcoin and 1 Bitcoin Cash.
Now imagine you sold the 1 Bitcoin Cash for 0.1 Bitcoin, you now have 1.1 Bitcoin!
Fast forward to the future, where Bitcoin Cash is the most popular one that is called Bitcoin, and the current Bitcoin is called Bitcoin Smallblock. Say your 1.1 Bitcoin Smallblock is only worth 0.2 of the new Bitcoins, but you want in, and convert them.
In this scenario, the fork-scum made you turn your 1 Bitcoin into 0.2 Bitcoin, without you ever wanted to sell them!
This scenario not necessarily going to happen, but keep in mind that Altcoins, ICOs and Forks are designed for only one goal: convince you to give your real money to whoever created the alt!
That is a good theory. Especially if the Bitcoin Cash had a deliberately lame launch so the creators could scoop them up for less than 0.1 BTC.