Because rabbits have a very delicate gut it is important to implement any changes to the diet gradually. Changing from one brand of feed to another should take a minimum of two weeks. Start when you have approximatly a week to 10 days of your current feed left. Using an egg cup measure out one portion of the new feed and add mix it with whats left of your old feed, then feed your rabbit from this for two days. Every two days add another egg cup of the new feed to the bag, after about a week the bag will contain nearly all the new feed...continue this process untill your rabbit is totally on the new feed and the old is all used up.
It is important when purchasing a new rabbit that you ensure you are given ample supply of its current food to allow you to change it onto your own brand. Make sure you wean all rabbits onto the same feed before you try to bond rabbits as unfortunatly trying to offer two different feeds to a group of rabbits can end in disaster.
Some feeds are notoriously difficult to wean a rabbit off (mainly any medicated brand pellets and rabbit royal... unfortunatly both varieties are often used by breeders.) If your new rabbit is fed on either of these varieties i would not commence changing the feed untill your bunny is at least 12 weeks old and then slow the changing process by adding a new egg cup every 4 days instead of every two, this is much more time consuming and you will need more of the origional feed (try asking if you can purchase a little extra when you obtain your new bunny) but is worth it in the long run, sadly i have lost rabbits when trying to wean off these brands in the past, so now i am always very cautious about any rabbit that has been fed on them.