Most such software does ">>" or O_APPEND or otherwise opens the file with different flags, so it would act normally. Otherwise you are correct that something would have to be done, I believe any call to lseek() could indicate that you want the previous behavior, and it would be easy to alter software to do this, or add some syntax to the shell to do this.
Also it is possible that the file will revert to previous behavior after a certain size is achieved.
I still feel the benifits outweigh any compatibility problems. By far the majority of programs writing files with creat() act as though it works exactly like I describe.