I changed my horse's name without hesitation. It was Tena. She is a grey flea bitten QH. I named her Greycie and she has always responded to it. I don't think she liked the name Tena either! LOL
I also had a mare when I was in highschool (many years ago) that my dad bought for me. She was a morgan/QH cross and wasn't registered. He didn't remember what they had called her, so I named her Perfection. She also did just fine with her new name.