Converting struct to unique_ptr from struct member

Question

So in part of a larger refactoring, I need to assert that a certain struct is_standard_layout because of communication between server client. Consider the following:

struct CommonHeader
{
    uint32_t m1;
    uint32_t headerSize;
};

Previously all other headers had derived from this like so:

// old style
struct OtherHeaderOld : public CommonHeader
{
    // CommonHeader placed here implicitly with gcc compiler.
    uint32_t m2;
};

But this does not guarantee standard_layout. Until now the compilers we've used luckily placed the base as the first member. For the refactoring I'd like to do this:

// new style
struct OtherHeaderNew
{
    CommonHeader header; // placed here instead of inheriting. Same layout as old struct with gcc.
    uint32_t     m2;

    std::unique_ptr<CommonHeader> ToUniqueHeader() 
    {
        return std::unique_ptr<CommonMessageHeader>(&header); 
    }
};

So far, this should all be equivalent in layout. However, at some point in the code I need to transfer this header to a unique_ptr<CommonHeader>. Before this was easily done with:

// old style header here
auto uOtherPtr = std::make_unique<OtherHeaderOld>();

// do some init code here

// do implicit conversion is no problem 
// since it is derived from CommonHeader
unique_ptr<CommonHeader> uPtr = uOtherPtr;

// send the msg based on the CommonHeader
Dipatch(uPtr);

Now, with the new version of the header I'd like to do:

// new style header here
auto uOtherPtr = std::make_unique<OtherHeaderNew>();

// do some init code here

// no use of auto here to emphasize the conversion to CommonHeader
// this will compile, but will the UniquePtr clean up the whole object correctly?
unique_ptr<CommonHeader> uPtr = uOtherPtr->ToUniqueHeader();

// send the msg based on the CommonHeader
Dipatch(uPtr);

This compiles without a problem. So finally the question: Is this a legal way to convert to a unique_ptr?

Update Some people suggested that deleting via the first member was different than deleting the ptr to the object. I tried this with GCC here, https://godbolt.org/z/vzncsGbGY And it seems to produce the exact same output (try to switch with DELETE_VIA_FIRST_MEMBER)

P.S.
I tried to look at the documentation, but couldn't find out the appropriate place to look for the rules underlying this, so any reference will be very welcome.