I haven't seen this particular question, before. Non-disruptive in-chassis HW upgrade is not supported - it would get hung up on seeing mixed controllers in the chassis, IMO.
Thinking about it, it would have to be disruptive and you'd need to move any data off the FAS8300, first.
- Evacuate data off attached storage
- Unjoin FAS8300 (if applicable)
- Use parts of the "repurpose KB" workflow to init the FAS8300 and the attached storage (MSID 0x0 for encrypting drives, wipeclean, option 9a, etc.)
- Swap out the FAS8300 with FAS8700 controllers and recable
- Boot and init the FAS8700, create or join the cluster
- Move your data back.