Yes.They can just put more stuff on the stone?
. I did that with my sister last year. She was cremated. I had her put on top of my mother's grave, then added her birth date and death date on the back beside her name that was there ..along with the rest of us. Was trying to see if there was room to have the Manti Temple put on, but not enough room. Austin figured out there was enough room to put the sealing date. 

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