Aren't you able to mount the diskgroup because ASM use the ASM_DISKSTRING parameter to locate the disks and identified the member disks of the diskgroup by scanning the disk header.
If you are using ASMlib or UDEV rules, ASM disk names are independent from OS devices names.
I created Oracle ASM Diskgroup called VOL1 with the device /dev/sdb1 and VOL2 with /dev/sdb2 using oracleasm
Now the SAN LUN is moved and now the /dev/sdb1 is changed to /dev/sdc1 and /dev/sdb2 is changed to /dev/sdc2 (From OS /Storage side)
Now how can i change the Device name /dev/sdc1 for VOL1 and /dev/sdc2 to VOL2 so that i can mount these ASM volumes.
This is why your configuration and use of the I/O layer is wrong. One never uses the /dev tree directly for SAN devices.
Because SAN devices are NOT guaranteed to be consistently named as /dev/sd<n>devices. Device names can and usually do change after a reboot.
Because SAN devices mean that you are using a dual port HBA (fibre) or HCA (Infiniband) cards for connectivity - and thus have multiple I/O paths to the same SAN LUN. For example., SAN LUN0 will be seen by the kernel as devices /dev/sdg and /dev/sdh. Using a single I/O path is a bad idea ito load balancing and redundancy.
This is why a multiple I/O path device interface need to be configured, and used by ASM. You can use the proprietary device driver from the SAN vendor (e.g. PowerPath from EMC). Or you can use the Open Source multipath driver for Linux (recommended as it does not taint the kernel).
I suggest that you review your SAN LUNs on your server - using commands such as lsscsi to list the devices and scsi_id to uniquely identify the devices, and /etc/multipath.conf to configure these devices.
As for ASM - that should point its disk discovery string to /dev/mpath. It also does not store device names (so the actual name is not relevant). It reads the ASM label from the disk header to determine whether the disk is an ASM disk, what the diskname is, and to which diskgroup it belongs to. Failing which it considers the disk as a potential candidate disk.