Does a soldier have to wait for an in and out call to say yes in order to change an MOS?

There are many times and reasons a Soldier can reclass. The answer isn't as simple as yes or no. There are many, many, rules regarding reclass, and very, very limited class seats for MOS's that aren't application MOS's.

If you are an out of the reenlistment window Soldier attempting to reclass into another MOS, you must be at Overstrength, the MOS must be at Understrength, and there must be an available class date.

Most likely the MOS you are hoping for does not have class dates unless it is a very large MOS

Unless the reclass is due to medical requirements or some other extenuating circumstance, they will not take nor accept anyone if there is an N in the IN or OUT.