There is one tricky issue with going back and forth. On 4.1.2, you can view the true /sdcard directory. On 4.2.1, your SD card stuff should get moved to the /sdcard/0 directory. Here's where it gets confusing. On 4.2.1, the /sdcard/0 directory is displayed through root explorer (or any other file explorer) as /sdcard. So, if you still have any files remaining in /sdcard, you will not be able to view them on 4.2. On first flash on a 4.2 rom, this directory will be created and I think everything will get moved to /sdcard/0 automatically, so you won't have a problem initially, but going back and forth can be kind of a pain in the butt.