Installing 9500 watt, 30 amp generator. I have two 200 amp service boxes coming from the same meter. I plan to run the generator to one box and install an interlock device on that service box and relocate the needed circuits/breakers from the other panel to the generator powered panel and move the circuits not needed in a blackout to the unpowered service box. I understand I can extend the wires from each circuit to move them from box to box through conduit and need to move both the hot and neutral wires. I am guessing the grounds should also be transferred accordingly also. Any thoughts and input greatly appreciated.
You are correct, there is no way to feed 2 panels with 1 generator (except for some arcana using an isolation transformer).
Electric current flows in a loop. On 120V circuits, neutral is an equal partner in the loop (Even if its role is underplayed). Thus neutral must come over with the hot(s). This is unlike safety ground, which only handles current during (hopefully) momentary fault conditions. Ground can simply land in the panel that the Romex cable enters. Hot(s) and neutral must be extended.
When someone installs 2 panels next to each other, it is wise to install several short conduit pass-throughs between them. If this is done, then the circuit can be extended to the other panel simply with two THHN wires white and black. (Or 3 if it is a MWBC or 120/240V circuit).
Keep hot/neutral pairs identified or twisted together, and make sure any given circuit's hot and neutral go through the same conduit.