I understand your approach. And in cases of migration and change the name into the target language it would be useful.
But … In Germany you officially may give your child up to four first names. In noble circles they often give inofficially even more than four first names.
In former times the calling name was underlined.
First example: [[Barnscheidt-2]]
My 2-g-grandfather was Johann Heinrich Gustav Barnscheidt. His older brother was Johann Heinrich Wilhelm Barnscheidt. Their father was Johann Heinrich Barnscheidt. such a constellation is not uncommon in old German families.
So my ancester was the Gustav and his brother was the Wilhelm in the normal day family life.
The current fields
Proper First Name: Johann Heinrich Gustav
Preferred Name: Gustav
show the correct situation.
In the profile is correctly shown: Johann Heinrich Gustav (Gustav) Barnscheidt
So if you rename Proper First Name to First Name at Birth and Preferred Name to Current Last Name: It would not be 100% correct.
Second example: [[Grossteinbeck-1]]
Johann Adolph Grossteinbeck was born in Germany and emigrated to the nowadays USA. He americanized his name to John Steinbeck.
His names are entered as you mentioned. But the display on the profile page is not quite correct.
It shows: Johann Adolph (John) Steinbeck formerly Grossteinbeck
He was never Johann Adolph Steinbeck.
Correct would be: John Steinbeck formerly Johann Adolph Grossteinbeck
So it wouldn't be done with renaming the fields. There would something more be needed for complete name change.