I've been out a few years and most of my school still STRONGLY suggested or required science letters, especially if you've been out less than 5 years (also keep in mind that there are a few schools that require your pre-reqs to be completed with in X years of starting med school). I've heard of substitutions for non-science letters with employment letters.
My professors remembered me and were happy to write letters for me, but if you're concerned you could always ask for the letters now and host them on Interfolio until you're ready to apply.