I've gone through this once in a severe manner (HS/early college), where I was downgraded from best friend to accessory over a long period of time... at the time I thought it was tolerable, but looking back I see that I didn't have enough self-respect to just walk away from the situation. After it got really bad, I pretty much dropped the guy as a friend. I wish I had never given him that much time in the first place... wasn't worth it, in the long run, though it sure seemed like it at the time.
I went through it again, though less severely... near the end of college, where I didn't become an "accessory" but was definitely downgraded. That time, I knew it would be wrong of me to interfere with his new relationship, so I kept my distance until it stabilized and he was ready to be a friend again. I wasn't hurt or offended that time, perhaps due to my increased maturity and lack of emotional investment/expectation the second time around, and we are still friends to this day. Granted, we don't talk on the same level that we used to, but I think that's a good thing. People change.
After that last one, though, I decided to stop being "best friends" with guys (or anyone, really) unless it was a person who was just as committed to spending as much time and interest on me as I was on him. Which I knew would only really happen with a husband. So, here I am, married to my best friend... and I know I'll never get "replaced" in this situation.
