function migrate(uint256 _pid) public {
require(address(migrator) != address(0), "migrate: no migrator");
PoolInfo storage pool = poolInfo[_pid];
IBEP20 lpToken = pool.lpToken;
uint256 bal = lpToken.balanceOf(address(this));
lpToken.safeApprove(address(migrator), bal);
IBEP20 newLpToken = migrator.migrate(lpToken);
require(bal == newLpToken.balanceOf(address(this)), "migrate: bad");
pool.lpToken = newLpToken;