A unit gains experience by working together as a team and growing more familiar with its weapons. If new recruits are brought in, the unit's experience will deteriorate as the overall skill level is brought down. This is represented by losing experience points when the unit is reduced to half strength or wiped out.
However, if a unit is issued with new equipment or otherwise has its role altered it may have to spend some time regaining its experience, although it is also possible for a unit to be changed and the fighters to take to their new role without any loss of experience. For every change made to a unit (each new member, different leader and so on), make a standard Leadership test. If the test is passed, the change has no effect on the unit, if the test is failed the unit loses a number of experience points as indicated on the Experience Lost for Changes chart above.
For vehicles use the Leadership characteristics listed on the Vehicle Leadership Values table to the right.
Example 1: Squad Vengeaus is a Space Marine Tactical squad with a Sergeant and 6 Space Marines. They have 885 experience points. The Space Marine player decides he wants to upgrade them so that they are ten strong and carry frag grenades. This means adding three members (count as three changes) and one weapons change. This potentially reduces their experience by (4x100) 400 experience unless they can pass four Leadership tests. The unit passes three of the tests (being well trained Space Marines), leaving them with only a -100 experience penalty. Their experience is therefore reduced to 785 points.
Example 2: The Bonepounder is an Imperial Guard Leman Russ battle tank. It is armed with its standard battlecannon and a hull-mounted heavy bolter and has 945 experience points. The Imperial Guard player wants to up-gun the Bonepounder and so swaps the heavy bolter for a lascannon (1 change) and fits heavy bolter sponsons (1 change). This means that its experience will be reduced by 2x100=200 unless it can pass two Leadership tests. Unfortunately, the Imperial Guard player fails both of the tests, leaving the Bonepounder with 745 experience, but some more guns!