I was about to bring up the Illinois Central Mountains, but Rusty beat me to it. For IC at least, this upgrading from a 2-10-2 to a 4-8-2 was not just a small batch or test bed. According to Steamlocomotive.com, this modification was made to 55 ~ 60 locos.
None of the major manufactures have done a scale, mainline, steam Illinois Central engine. Not sure what the interest would be, but I for one would purchase IC mountain.
Charlie