Would it be okay to use "winter" formula of fertilizer in the spring.?

When you say winter im assuming that it is a slow release nitrogen to help feed the roots throughout the winter. Having said that if its the case, you probably see where I will go with this. What I would do is use it in the heat of the summer months, because in the spring you want to jump start the lawn quickly by giving it a healthy dose of fert. In the hot summer months when the grass slows down the slow release will help provide the nutrients without putting more stress on the root system that will be brought on by possible drought. But if you must it wont hurt it at all. Good luck.

Im giving you this advice as a business owner that does this everyday, do not use the winter fertilizer in the spring, you will burn your grass and slow its growth process, it should only be used a month or two before winter.

fertilizer is just fertilizer so it makes no difference when it is used, percentages of elements might be different in what you have than what is normally used but its still fertilizer.

Absolutely. Use it up because by next fall it will be no good anymore.

probably not