kouisitu de seihuku ni kigaeru koto ni natte imasu.

In standard modern Japanese, it should normally be 着替える. So the sentence would usually be written:

更衣室で制服に着替えることになっています。

着替える is the dictionary form of the verb meaning to change clothes or to change into something.

V-ることになっています means it has been decided that..., it is مقرر / arranged that..., or it is the rule that....

So here, it does not sound like a personal choice. It sounds like an established rule, procedure, or arrangement. In context, it means something like:

It is the rule that you change into your uniform in the changing room.

Because the verb comes before こと, and before こと you normally use the plain form.

So:

• 着替えること = changing into... / the act of changing into...
• not 着替えますこと

The sentence becomes polite at the end with なっています.

こと turns the verb phrase into a noun-like expression.

So:

• 制服に着替える = to change into a uniform
• 制服に着替えること = changing into a uniform / the act of changing into a uniform

That whole chunk can then be followed by になる, giving the idea of it has been decided that changing into a uniform will be done.

で marks the place where an action happens.

Since changing clothes is an action, the changing room is marked with で:

• 更衣室で = in the changing room

If you used に, it would not sound right for the location of this action. に is more for destination, existence, or a point in time.

Here, に marks what you are changing into.

So:

• 制服に着替える = change into a uniform

It shows the result of the change. You start in one set of clothes, and the result is being in uniform.

They are related, but not the same.

• 制服を着る = to wear / put on a uniform
• 制服に着替える = to change into a uniform

着替える includes the idea of switching from one outfit to another. 着る only means putting on or wearing the uniform.

In ことになっています, the ています shows the current state resulting from a decision or rule already in place.

So this does not mean something is currently in the process of becoming decided. It means:

• the decision has already been made
• and that decision is still in effect now

That is why ことになっています often means it is the rule, it has been arranged, or it is set up that...

Japanese often leaves out the subject when it is clear from context.

So this could mean:

• you are supposed to change into your uniform
• students are supposed to
• employees are supposed to

The listener is expected to understand who from the situation.

This is a very common question.

• ことになっています = it has been decided / it is the rule
  
  • usually an external decision, rule, or arrangement
• ことにしています = I have decided to do / I make it a rule for myself to do
  
  • usually a personal decision or habit

So in this sentence, ことになっています suggests a school or workplace rule, not the speaker’s own personal choice.

Yes, aside from the spelling of 着替える. With that corrected, it is a very natural sentence for explaining a rule or procedure.

A natural standard version is:

更衣室で制服に着替えることになっています。

It sounds like something you might hear at a school, workplace, or event where there is a dress procedure.