I switched from Coumadin to warfarin several years ago with no problem in INR results. However, I have found, over the past few years, that there can be some difference between brands of warfarin that can affect my INR. I now use only one brand and seldom have any INR problems. My pharmacist agreed that it is best to stay with one brand as there are acceptable tolerances between drugmakers. My own experience is that vit K foods have the biggest impact on my INR. I try to have some "greenies" most every day and that helps keep me in my range of 2.5-3.5.
FWIW, there are two warfarin brands in the US that have been around for years....Barr Labs and Taro. I have used both with no problem, but have settled on Barr Lab for consistency. I had a "big box store pharmacy" substitute a little known foreign brand that caused me a great deal of INR problems. Once I stopped using that brand my INR settled down.