Federal Reserve officials will consider a modest but symbolically important change in the management of their massive securities portfolio when they meet next week to ponder an economy that seems to be losing momentum. The issue: Whether to use cash the Fed receives when its mortgage-bond holdings mature to buy new mortgage or Treasury bonds, instead of allowing its portfolio to shrink gradually, as it is expected to do in the months ahead. Any change—only four months after the Fed ended its massive bond-buying program—would signal deepening concern about the economic outlook.