If you are going to be switching to grower (17% protein), you can still mix in your starter (20%) half and half, or something like that, to use it up.
Commercial cracked grains will keep under ideal storage conditions no longer than 3-4 months. I wouldn't attempt to store it that long in summer heat. The vitamins break down and it is best to use it up quickly for summer.
How old are they? You can feed starter up through 8 weeks of age I have read (and do it myself). Some feed is a starter/grower feed, designed to be fed until the POL (point of lay).
If you want to know what I personally would do, it would be to mix in scratch grains to bring the protein level down to 17% - and I'd make sure green grass or other fruits and vegetables were available at all times for vitamins (since you would be cutting the vitamins of the starter). I actually feed organic unmedicated chick starter and other seeds and grains (with oyster shell and grit) to my layers.