I am pretty sure you can change/add lifecycles anytime but they will only be applied to the vaulted files as they are being re-checked in. You may be able to use the API or a macro to check the files out and then back into the vault with the lifecycles being applied. Someone may already have a macro to share?
Just my 2 cents... consider including an "obsolete" type life cycle in your mix. That will cover "cradle to grave" for your items.