The available online help will give you tips on what to do. If you're not sure, don't change it. Interdependencies are sometimes delicate and hard to grasp. Better a kernel that is some kilobyte larger than strictly necessary than one that doesn't work at all. Next save your new configuration.
Compile time depends on system and kernel configuration, of course.
These 'files' are actually links. Leave them in place and trust M's init script to sort things out, i.e. to change these symlinks according to the kernel you are booting. Delete them and change your bootloader config file to reflect the exact kernel versions you are booting. You can choose an arbitrary name for the image, as long as it is different from the old one. You then have to tell either LiLo or GNU GRUB about the image.