We are unable to convert distribution groups to security groups via EMC or EMS.
The groupType attribute is the one which is controlling the security and distribution "Group type" for any group-object.
The values are:
a) For Distribution "Group type", the value is: 8
b) For Universal security group: -2147483640
For global security group: -2147483646
So, If we can change the value of "Group type" via script, to convert distribution groups to security groups in bulk may be achieved.
In addition, I would like to know why you need to convert distribution groups to security groups. Because of some Exchange attribute of the distribution groups may not change to the attribute which security groups have.
The msExchRecipientDisplayType attribute of Universal Distribution Group is 1, the msExchRecipientDisplayType attribute of Universal Security Group is 1073741833.
Even if you changed the Distribution Group to Security Group via ADUC, but the msExchRecipientDisplayType attribute is still 1.
Therefore, in some scenario, Exchange server still recognize it as Security Group.
As this point, to recreate the groups as Security Group is recommended.