After December 31, 2009, mid-efficient forced air furnaces - with an efficiency of less than 90% AFUE - will no longer be available. When your old furnace needs to be replaced after this deadline, it can not just be replaced with a high-efficient unit of the same size.

Your existing furnace is probably a "standard size" furnace, operating with a 50 C (90 F) temperature rise, with a "standard size" duct system. Modern high-efficiency furnaces operate with a temperature rise around 30 C (50 F) and about 40% increased airflow. Thus replacing your existing furnace size by size would lead to noise complaints.

Before selecting a new high-efficient furnace, your contractor needs to perform a "shell" heat loss calculation to determine the required equipment sizing. Most likely the resulting reduction in furnace size will be found compatible with your existing duct system and will only require minimal changes.
Critical is the set-up and commissioning of your new high-efficient furnace, including venting. It is up to your installer to perform the set-up such as to ensure maximum efficiency and comfort for your local conditions and record the set-up for future servicing of your furnance.
Chose an experienced contracter who will get it done right the first time, and don't forget to also have your Ductwork cleaned before enjoying your new high-efficient furnace!