Keep the proportion agreed in the contract unchanged. Note that I switched the two sentences of the question.

Because I think the reduction of B shares is due to the reduction of A shares.

Graded funds have three different types of shares, namely, parent funds, Class A and Class B. The net value of parent funds is generally linked to an index. Class A funds get fixed income every year, and Class B funds are equivalent to adding leverage to the index corresponding to the parent fund. The net value of the three funds conforms to the weighted average ratio of A and B as stipulated in the fund contract.