I now call all my aunts and uncles by their first names. When I was younger it was "auntie/uncle name". My grandad on my dads side I've always called "grandad name" but my other grandad, granny and nana are still just that. My DD has a nana (DH mum - same as her mum was called), pops (DH dad - same as his dad was called), granny (my mum - same as her mum was called), grandad (my dad - same as his dad but without the first name at the end) and nanny liz (my step mum).
Maybe ask your DHs parents if they would be comfortable with nana/grandad and then their names?