bonus data will still be there
but if you are moving to 5G plan, since 5G has unlimited data at 512 Mbps, those bonus data will became incompatible addon and cannot be used while you are on 5G plan. They are still in your account, and will become "compatible"addon and can be used again, if you go back to 4G plan one day
I have some bonus data add-ons that I guess I will be losing if I switch to a 5G plan. I have a 1GB, a 2BG, and another 2GB bonus. I also have lots of international and US long distance minutes. Hopefully I get to keep them.