Hi JC,
I've had the same dilemma as you since I bought my '53 plate A3 a few weeks ago, aren't the new grills just sooo much nicer??

( I personally prefer the back end of my car to the newer ones though!)
I searched around on forums with no luck, I can't believe more people haven't considered this! Anyway I finally stumbled accross a few solutions... but none of them are tested yet!
Firstly you could replace the whole bumber with an aftermarket alternative such as:
found
here. I didn't go for this option, mainly because there is a lot of work involved with priming, painting and fitting, but also because I don't want to mod my car too much for insurance reasons. Plus I didn't really like the look of any bumper replacements I found, and I'm sure loads of people on here will tell you to keep your car OEM. I'm all for personalising your car, but apparently it's not the way to go!
The second option I found was much cheaper, and easier but just didn't do the car justice...
found by searching for "Audi A3 8P singleframe grille chrome" or similar on eBay. These look 'goodish' on the pictures above but I'm not sure how good they'd look close up, and if you don't have a black car( like me, I have dolphin grey) then you're going to have a coloured strip within the grille anyway, which spoils the illusion, along with the horizontal chrome bars of the original grille.
Finally, the option I chose was this:
That was exactly what I was looking for! It's a great looking solution, requires virtually no work at all and is relatively cheap compared to the first suggestion!
It's made by a German company called ABT, but can be bought in the UK from
here. Just select your car and browse the 'styling' section!
I'm still waiting for mine to be delivered, as of an order placed over a week ago now, but when it does arrive I'll give an opinion but probably not a fitment guide until after Christmas, if I bother at all!
As a final option, apparently Audi offer their own singleframe conversion for our grilles, but at a higher price than the ABT. I didn't look further into this,I only read it
here.
Well I hope this helped someway towards answering your question! Can I suggest you rename the subject of the post to something along the lines of "Audi A3 8P split-frame to single-frame grille conversion options" I know it's long winded but it will create better results for people searching for it, and may inspire people who are just browsing the forum! It's up to you though.
As I'm on a picture posting run, here's my totally stock car, which it won't be shortly after christmas!
Merry Christmas everyone!
Wes