That’s a very good point, and I would never remove the old parts. That’s the beauty of Dorico, it really is very easy: once I’ve written the parts in their original transpositions, I can do the “modern” transpositions with a handful of clicks as additional layouts. @Craig_F showed me how.
I’ve seen several modern orchestral arrangements that provide several parts to choose from, for example Éditions Marc Reift, and that’s kind of what I’m aiming for here. In the case of the orchestra I’m doing this for, these are all amateurs who could transpose the parts themselves, but probably not by sightreading.