Rainbowz.
I think it's a wonderful idea. I made up my last night. I was deserted by my family at a time when I desperately needed them. So, I legally changed my name.
The only problem I've had is that I have a Traumatic Brain Injury. Sometimes when I've had a really bad relapse, I can't remember my name. I'm sure that won't happen to you, though, and I think it is a wonderful way to honor your beloved grandmother.
Hugs,
Jan