I did a guts replacement on our 1950's Cutler Hammer CH series panel a few months ago. It was an older style 200amp panel with a split bus backplane. Replacing the entire panel would require redoing the outside brick, a new meter housing and on and on.
So I contacted the inspector and told him I was planning on using one of Eaton's backplane replacements. I managed to find one that was only slightly larger than the original. All I had to do was drill one hole for the top mount as it was offset. Then add the main breaker block off as we have a separate disconnect for the main box and an extra grounding bar.
Inspector came, gave me a thumbs up on the retro fit and signed off. By speaking with the inspector and letting them know what I was doing, he was fully prepared when he should up to inspect it.