The style of connection between pads/vias and a powerplane or poured copper (isolated, thermals or not-isolated) can be set for an individual item from the Properties dialog. Setting it on a more global basis currently requires the addition of a 'rule' for a net class. The procedure for Vias is described below, but can also be applied to pads (surface-mount and through-hole).
Creating the rule
Select Technology, then DFM/DFT Rules.
At the top under Rule Level, click Net Class and choose the required class from the dropdown list.
Under Thermal Pads, check the box Net Class Override.
Now click Additional Rules.
Choose Vias from the drop-down list, and click Add.
Under Rule, choose the desired connection type from the dropdown list.
Click OK twice to close the dialogs.
Now when you pour your copper or produce your powerplane plot, the selected connection type will be used for all vias on the selected net class.