Windows clustering relies on disk signatures so drive letter swapping would not work. I would shutdown application/sql, shutdown other members of the cluster and just leave one up, robocopy data from E:\ to I:\ , delete E:\ physical disk resources, recreate it but point to drive letter I:\. Always have a fresh backup copy available

Open Replicator will make things easier for you , do you have it ?

The cluster does indeed rely on signatures to identify the disk resources - however you can change the drive letters. I would recommend the following

1) disable SQL services in services

2) remove all dependencies on the 2 drive letters in cluster

3) remove the disks from cluster

4) disable cluster and reboot on all nodes (one by one)

5) change the drive letters at the OS level.on all nodes (one by one)

6) re-enable cluster and reboot

7) add the drives back in assigning the 'new' drive letters

8) test failover etc.