It is clear how badly Gregor Samsa wanted to be seen as someone who could take care of his family, and more sharply, how badly he wanted to be recognized for it. This is the spiritual metamorphosis: his slow release of the status and self-esteem he (sure, deserved, but more importantly) demanded credit for. More than being loved by his sister or parents.
The cockroach body stripped away most desires at once, but not this one. This took time. Gregor was so hard wired to work for money and being a slave to the pecking order at his office that only incremental horror could dissolve his need for reward. He slowly gives up on the idea of being credited for all the good he had done. That caused his ego-death.
This is my first time getting introduced to Franz Kafka through "The Metamorphosis"—the English version. I haven't read his other works, but I've heard he puts characters into absurd situations to expose what's so usual and fundamental in day-to-day life that we operate on auto-pilot mode. Coming back to this book: Gregor, incapacitated, wants his sister to "hold the chief clerk from leaving" — his only shot at keeping his job. This nature of Gregor later contradicted itself when he watched his sister not being praised for playing violin in front of the tenants. Both situations resulted in financial loss for his family, yet the latter change of behavior points to his partiality in matters that directly concerned him.
There is that constant desire for validation Gregor seeks "like feeling great pride in the fact that he had been able to provide such a life for his parents and sister in such a nice apartment" and that "money was gratefully accepted and gladly given" so much that "they got used to it" but "there was no special outpouring of warm feeling." Gregor was envious of the attention his sister was getting after his ordeal. She became the "expert" who knows what's best for Gregor whereas formerly she was "thought of a somewhat useless girl" by their parents. At times, he was not bothered about his family and just filled with rage at the way they neglected him.
This pride, this self-image Gregor wanted to build in this world of slave culture—running after fame and money, seeking validation from people around him—was brought to a halt slowly when realization began to strike him. The anger and sorrow faded away in want of real human connection he'd been missing for so long. Empathy seeped in through the cracks of turmoil. He started to accept his fate and just let go. Letting go of his need for validation, his need to be the provider, his need to be seen as valuable. This was the spiritual metamorphosis of Gregor.
Now, many people have different takes on what Kafka wanted metamorphosis to mean. These are my thoughts, opinions, whatever you want to call them. I like Ralf Sudau's take on Wikipedia; our theories somewhat overlap. But the particular attention I want to emphasize remains this: Gregor's tragedy wasn't that he became a bug, but that becoming a bug was the only way to kill his desperate need to be seen as the provider.