Nothing ventured nothing gained. Shouldn't have any issues with it, you might need to tap the disc with a hammer. Take it steady and you'll be fine. Don't forget if you're changing a disc you'll need new pads too. Always replace in pairs too.
find the best fitting screwdriver you can find, hold it in place and apply a little anti-clockwise tortion with your left hand, and then give the screwdriver a tap with a hammer using your right hand. Impact is the answer, it is very easy to round off the screw head otherwise.
Impact screwdriver is a good tip, I bought one for £12 from Halfords for exactly this purpose. I assumed you didn't have one, so remember: Screwdriver + Impact = Impact Screwdriver (If you can't be arsed to buy one)
The screws are usually seized in - an impact driver will almost definitely be required. Be prepared to replace the screws with new ones as they are only soft and will usually be destroyed during removal.
My wife's A3 did 50k miles without this little screw after I drilled it out when changing the discs. AFAIK, it only holds the disc in place when the wheel is off and when the wheel was off I never had a problem.
