I bought a full set of Kaplan books used a two years ago. It is made for the current test format. Should I still use this or should I just invest in an updated version?
You can always use AAMC content outlines and Khan Academy videos for anything you might have missed from your Kaplan books. But you should be fine using the older books
I'll slightly rephrase Lawper's suggestion; you should be supplementing Kaplan with other resources, but you aren't missing out by using an older Kaplan versus the latest edition.
I think the biggest difference I've found was at one point they started highlighting sections as high yield or not which might be worth it to you but probably not.