Now, I am no Tebaldi specialist, and have not listened to all she had to offer. I mostly limit myself to her early, live recordings, and some of the early studio recordings in which the voice sounds fuller and less harsh. But sampling Tebaldi throughout her career, the first thing that I hear is the loss of what one could call the "bloom" (cannot find a better word) that she had in the beginning. What I mean by bloom is the "ooooooh" sound (like an owl) that you hear when she sings, even when there is a lot of squillo, like in this performance:
In this performance, the voice is not harsh nor does it has this edge that would soon creep in. The bloom is like a cushion on which every notes sit. The feeling I get is that it enveloppes the voice. I think you hear it more easily with lower voices, like in this performance of
Ave Maria by Christa Ludwig. Listen to how the "ooooooh" sound is always there, whether on the low or on the high notes. Listen to how she sings the vowel and how she slightly changes them, enough to maintain the falsetto action, but not so much that we cannot make up what she sings. Ludwig wrote about that in her autobiography and how changing the vowels without it being obvious was an art in itself:
Returning to the Tebaldi video, The "bloom", which is the opposite of harshness and edge, was present throughout her range and is only possible with a proper coordination of the registers, and more specifically proper falsetto action. To maintain proper registration and falsetto action, one has to sing dark and with proper resonance in the pharyngeal space. As a singer climbs up or down the tone scale, he should maintain such resonance by shifting the registers and slightly changing the vowels. As you go up, there should be more "ooooooh" in the sound, in order to maintain the throat open. But there should still be chest voice, in order to maintain clarity and squillo (a singer cannot really control these things, but he or she should try to obtain the right sound whilst maintaining the voice free of any undesired tension).
Just like Callas, I believe a "de-coordination" of the registers brought about Tebaldi's vocal decline. She lost that dept in her sound, which comes about when proper falsetto action is failing. She therefore developped an edge in her sound, and her high notes became shrill and flat, like screaming (not all the time though). You can also see in some videos that she sometimes sang with her mouth barely open, and that she would sometimes spread her mouth to reach the high notes, which naturally tenses the throath and make you lose the necessary depth to hit the note on pitch and with said bloom. The matronly tone also started to appear. Now her late recordings, like
La Gioconda, are not complete disasters, and she could still spin some beautiful lines. But the changes can still be heard easily. To conclude, I think this video does a good job of explaining what became wrong with her singing, and finishes with a great display of what she actually did so well when she had freshly arrived on the operatic stage:
But to answer your question more explicitly, her reworking of the voice made no real difference for me. It did not really bring back what was so good about her singing. It may not have been her fault, for the knowledge necessary to solve her problems was getting rarer. It seems to be mostly forgotten today...
