[protobuf] Is Resulting Binary from Changing Proto Definition Source (From Nested to Non-Nested) for Fields initialization Backward-Compatible?

['Felik' via Protocol Buffers]

Hello,

I came to understand that the binary serialization only concern of field 
identifiying number (hence renaming field and message is backward 
compatible).

I would like to confirm my understanding if following case is also backward 
compatible (I could not find any resource that discuss about this, or I may 
have used the incorrect search term for this).

Let's say previously I have defined a nested proto definition as follow:

message OuterProto {
   message InnerProto {
      optional bool field_bool = 1;
      optional int64 field_int = 2;
   }

   optional InnerProto field_composed = 1;
}

Now, I would like to "un-nest" the InnerProto definition by redefining 
another duplicate proto in top level as follow:

// same exact field numbering & type
message NewInnerAsTopLevelProto {
      optional bool field_bool = 1;
      optional int64 field_int = 2;
}

Would following modification to message OuterProto be backward-compatible ?

message OuterProto {
   message InnerProto {    // unused
      optional bool field_bool = 1;
      optional int64 field_int = 2;
   }

   optional NewInnerAsTopLevelProto field_composed = 1;
}

Any input & insight would be much appreciated. Thank you!

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