i want to change the subnet mask

I was in the same situation last week and only way at-least from my perspective is to delete and recreate the self ip's and floating ip's. If its just the IP address then you can update the bigip_base.conf and do a load /sys config...but changing the mask with out deleting is not possible. The impact is the virtual's on that vlan will be down until the new self/floating ip's with new mask are created. This was the case when I was updating it anyway.