You create a child style of the dimstyle you want to use.
So open dimstyle manager
highlight the dimstyle to modify
pick the New button (I know it doesn't sound right but trust me)
in the dialog that pops up to to the lower selection and pick the down arrow
Pick the diameter dims choice
pick continue
on the Symbols and Arrows tab pick the arrow for leader, note some things are grayed out since you are creating a child style for diameter
pick OK,
pick New and repeat only for Radius dims...
Finally pick close
Try it out with that style set current.
GrantsPirate
Piping and Mech. Designer

Always save a copy of the drawing before trying anything suggested here.
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If something I wrote can be interpreted two ways, and one of the ways makes you sad or angry, I meant the other one.