If the major of the library does not change, an update to the RHEL7 version might be feasible, but it will pull some more dependencies as mentioned already. You could test that within a clone of the machine where you try to upgrade this. Keep in mind that your support subscription might also be rendered invalid by this - either you run RHEL6.7 or RHEL7, I suppose a mix is not supported by Redhat.
Another option would be to download the src.rpm for krb5, and rebuild it locally - but the same as above applies.
Easiest is probably the installation to an alternate prefix as Popieluch already suggested, as this can be done in parallel to the system version. Just keep in mind that you need to point the applications you want to link against krb5 1.12 explicitly to where you installed krb5 1.12.