One more thought RP-
I know for me-my parents love my children-but the truth is we have a young child and older parents. If you did want to change it could you not address it in a way (fits with what I said above) that noted that due to their age you would not prefer they be in the parental role, but someone more your age and likely to be around longer-but that visitation with them be MANDATORY for the new person to ensure between child and grandparents?
Hard choices yes.
I remember when we had Ailsa-(my oldest) this was VERY important to me and I was adamant it not be grandparents for the simple fear that she could lose me to an accident-be put with a grandparent and lose them to "natural" causes. Too much for a child to go through.
"Love As Thou Wilt"