You misunderstood. Converting by share means converting into amount first, and then transferring money.

Convert 2000 copies of 1000 of Fund A into Fund B.

If the net value of fund A is 1 yuan and the net value of fund B is 0.8 yuan,

If you don't consider the cost. That is 1000 yuan /0.8 yuan = 1250 copies.

You convert 1250 fund b, 1250*0.8= 1000 yuan.

Your money is still the same.