It depends on how good is the vintage transformer. For ex. I have UM57 that is completely healthy, but the transformer makes it distorted/narrow/midrangey. That's where you don't want to keep original xformer.
I also prefered sound of the old xformer in comparison to tab-funkenwerk... but my opinion is based on some samples from internet (cough cough) so take it with huge grain of salt. But what I liked on the old was certain color/glue that was absent in the TAB sound. TAB sounded nice, but it was a different mic, way more normal and I don't need that kind of sound
. Not from CMV.