I am not sure what you mean by try reassigning material to the board after importing it to Icepak?
I exported the 3D diagrams from the Altium EDB exporter, so I should have the rights to those components. I have assigned them various materials as shown in one of ANSYS step-by-step simulation tutorials. I imported my EDB file through the ANSYS 3D HFSS and checked to make sure all the stackups/materials matched my Altium project before importing them into the IcePak project. I have attached my different materials for my project. The secondary solids starting with Air(@300K) are all of the component materials I am using, and the solids under the GS61008T are the PCB stackup materials.
Do you see any issues with the current materials being used?