Apparently you can upgrade without affecting your feed in tariff as long as you don't touch your inverter. I am getting mine upgraded at the moment and I am also on the 47c feed in. I've got 3.7kW worth of panels on a 4kW inverter. I am keeping my inverter and increasing my panels to 4.9kW.

Things to keep in mind... (from talking to various solar companies)
1. If they touch your existing panels because they need to rebalance/redesign your system, then they need to check and bring those panel installs up to current regulations (which is usually done at a cost to you)
2. You are not allowed to exceed your inverter capacity by more than 25%. So if you have a 4kW inverter, you can't exceed 5kW of panels. That is apparently a new requirement by Western Power/Synergy.
3. Try to get the same make/model of panels as your existing unit. If you can't get it, try to keep it to the same wattage. So if you have 200W panels, try and get 200W panels. This is very important if you are connecting your old panels to the new panels. If your inverter has a spare MPPT tracker and the new panels are going to be installed on this tracker, then you can get whatever panels you feel like.