I could've written your exact post a few months ago (actually i did do one very similar. I went back to work 9weeks ago (3 days a week though my mum is looking him the 3 days until 2 of the days become available at childcare....still waiting)- DS2 was just over 11months old and pretty much BF except i'd already introduced formula a few months earlier cause i was doing some 'me' time 1 night a week so my mum or DH would give him a bottle before bed. S26 was DS2's preferred formula but DS1 seemed to only like Karicare Gold. But leading up to DS2 turning 1 i was starting to introduce cows milk which seemed more of a hit then formula- so the last tin of formula was thrown about 2 weeks after DS2's first birthday(and it was a similar age for DS1 too). We also found sippy cups worked better too.

As for BF- we continued to do it morning, evening and through the night- DS2 has only just started to work out the whole sleep through thing. 2 months on and still going strong.

So yeah...it can be done. Good luck. Hope you have the same success as we've had (which i didn't get with DS1).