I migrated a stack of 9300's in my lab and it was fairly simple, but it takes a long time for them to come up in a usable state to configure the management vlan and IP on the management port. The switches rebooted multiple times over a couple of hours before they were finally stable. Going to be a pretty big maintenance window to upgrade to a supported image and then have several hours of downtime before you can even start configuring them.
A few other issues to note. There's no port profiles for the C9300's so you have to configure the ports currently. The C9300's list a bunch of extra switchports that don't exist. IE I have C9300UN's so it has the 48 ports, 2 stacking ports, and the NM-8X. But the switchports configuration lists 48 ports, 2 Stack, 2 C3850-NM2-40G's, 4 C3850-NM4-10G's, 8 C3850-NM8-10G's, 2 C9300-NM-2Q's, 8 C9300-NM-8X's, 2 2x40G, 4 4x10G, and 8 8x10G ports.