Hi Sagar,
You can definitely use the 'Change Key Figure Settings' option in SCM 7.0. I have used it to make a KF fixable and zero allowed. Even with this process, the best direction would be to perform it in Dev and Quality environments to see if you have any issues that show up. As a contingency plan, I would recommend taking a backup of all transaction data for that planning area just before performing such actions.
Regards
AK