SG-VA 6.7 for Hyper-V does not support upgrading number of CPU cores.
SG-VA 6.7 for Hyper-V
There are two reasons why it cannot be resizing.
SG-VA 6.7 for Hyper-V supports only C1 and C2 model. It indicates it cannot be resizing to larger than C4 model. Another reason, it cannot simply be supported for resizing in SG-VA 6.7 for Hyper-V edition.
Please use 7.3 to upgrade. 7.3 supports four CPU cores and more. 7.3 can resize resources such as CPU, memory and disks. Refer to see the description.
Resize the ProxySG VA on Hyper-V