I have a combined network that I need to split in order to upgrade an MX100 to and MX450. I need to clone the configuration of the MX 100. If I split the network while it is active will any configurations be lost? Will clients be disrupted?
No. If you split the appliance network out of the combined network there's no impact.
If you're just doing an upgrade though I would remove the existing MX100 and then add the MX450 instead of messing around with two networks. You'll have to adjust the physical ports configs, and deal with cabling, but I think it's easier than trying to clone the network.
Why is the title of your post, "Client VPN split tunneling?"
I have split combined networks and combined them again and not seen any service interruption or other trouble. I did this most recently to get rid of an old demo MC network I had in one of my combined networks. If anything, you *might* lose historical data like client usage. If that is important, I would call support or research more to find out.