Switch disks on a SAS 6/iR controller RAID1

"Can I just switch physically the primary disk with the secondary, so the new disk will be the primary disk?"

There is no primary disk  on lsi based controllers. The is a preferred disk the controller writes to first, it could be either disks of a raid 1 pair, chosen by the controller upon installation.

 Can I just switch physically the primary disk with the secondary, so the new disk will be the primary disk?

If you were to "switch" the disks, it is a bit involved, no less there is absolutely nothing to gain, the data is the same on both disks.

"Does this controller read from the primary disk or from both?" There are raid controller which read from both drives in raid1, LSI based do not, the controller will read from the disk which is preferred by the controller, unless the is an issue, then it will read from the other disk in the array.

The controller picks one of the disks, by parameters I do not know, speed is likely the main one. That said, it is not worth changing a disk for this reason. If you were to benchmark both drives, they could have the same exact results, and still the raid adapter would find a preference to one of the mirror pair disks. Besides, reversing the disks is not just a matter of physically moving them. In years past I benchmarked a good bit, and did as you proposed, there was no difference in benchmark results after the switch of drives.

The only time I do this now is if I "pull" a disk ( and add another to rebuild) for an emergency backup, and actually have to restore from the pulled disk.

 "'it is a bit involved'"  You would need to remove a drive, boot with the remainder, add a drive back in; the controller may or may not add the drive back unless a foreign config is cleared.