This is a FAQ on the BAA website:
I will be transiting through your airport. What happens to any tax and duty-free shopping (liquid purchases) that I have purchased at my departure airport (EU community country)?
So long as your shopping is intact and you have proof-of-purchase within the 24 hours prior to this, you will be allowed to carry your purchases onto your next flight. However, if you ‘land’ yourself (that is, you go through passport control and the baggage hall), you will not be allowed to take those purchases back through security and so will have to pack them in your hold luggage.
In addition, the Changi Airport website has comprehensive guidlines for worldwide travel and is worth a look:
http://www.changiairport.com/changi/...ml?__locale=en