They are separate breeds now...but that wasn't always the case.
"The Norwich and Norfolk Terriers were considered the same breed until 1964 when two different classification were created based on their ears. This separation occurred in England in 1964, but it took until 1979 for the U.S. to separate the breeds."
"Both the Norfolk and Norwich were born in the same litters at the time, and both prick eared and drop eared varieties emerged from the same whelp." Norfolk Terrier Puppies Breeders Terriers
There are many other examples of a breed where standards were changed and variety classes were granted because breeders preferred a different look, size, color, or coat texture in their respected breeds.
The dachshund is another breed. It has three variety classes for coat.
1. smooth coat
2. wire coat
3. long coat
The beagle allowed breeders a variety class for 2 inch's of height. Both varieties share the same standard except for height.
We already talked about the cocker spaniel in this thread...they have 3 color variety classes. Parti being one of them. Their buffs, blacks, chocolates and roans sure haven't been damaged because of the parti being added to their standard.
Everyone knows the poodle come in three different sizes and are placed in three different show categories. They too have a parti that is AKC registered and can now show thier "multi-colored" poodle with the UKC.
The smooth and rough collie share the same standard except for coat and have their own variety class.
The Pembroke and the Cardigan Welsh corgi's were once the same breed...and were separated because some breeders wanted a docked tail.
These are just a few examples off the top of my head..I am sure their are plenty more.
The breeds listed here who allowed changes to the standard and added variety classes have not suffered because of it. Nor as someone falsely stated earlier in the thread...the original color's or trait were not bred out. Scare tactics are old school and boring. IMO that was a frightened and uneducated statement. The parti gene has been around for many, many years in other breeds, and they still have their original colors showing, still breeding true, and doing quite well.