Replacing the GUID with an integer, especially on the cluster, is a good idea, but I would not go through that interim step of moving it to varchar first. You're taking a 16 bit key and turning it into a 36 bit key. That's not going to help your performance, in any way. A better approach would be to add a new column, as suggested above, int or bigint, and then migrate the keys. This will result in less down time than trying to change the data type and it won't hurt performance unlike the varchar route.