There is nothing wrong with using the Starter Fertilizer with Crabgrass Preventer on an existing lawn. While the nutrient analysis is best for new seedlings, it will not harm your existing lawn.

Thanks. I plan on using 12-24-12 Starter Fertilizer + Siduron.

What kind of grass are you planting? If you're planting perennial rye, spring seeding is fine, but if you're going with KBG or tall fescue, dormant seeding is a better option. To dormant seed, wait until it is too cold for the seed to germinate and spread the seed. I like to do it just before the first good snow storm of the winter.

The snow melting and the freeze/thaw cycles bring the seed into contact with the soil. Once it is warm enough in the spring, the natural moisture from snow melt and spring rains should be enough to get germination. Dormant seeding usually gives you a head start of a couple of weeks on spring seeding.