Hmm. My answer is based on my best guess at interpreting what you just said.
1.) Assumption. Debasing = opponents models in base 2 base contact were killed before you were able to swing on them.
All right now that it is out of the way, lets proceed.
Everything I am about to say is found on page 35 of the BRB.
Units that have one or more models in base contact with enemies are said to be 'locked in combat'. Within such units, the following models are said to be 'engaged' and must fight.
* Models in 'base contact' with enenmy models.
* Models within 2" of at least one model in their unit that is in base contact with any enemy models.
All engaged models will fight in this turn's Assault phase with their full number of attacks they have.
Working out which models are engaged in combat is done at the start of the fight, and will not change until its end, but casualties may make it difficult to remember as the fight continues, especially in a large combat.
As you can see, everyone that was elgible at the start of combat gets to fight, even if they are no longer in Base 2 Base later on in combat. Its up to you to remember which unit they could attack though.
The only exception I can see to this is if all elgible units that you could have attacked are wiped out, then you wouldn't have anything to attack, but it would have to be the entire unit.
This happens more often then not to IC vs IC fights where some other unit clips one of the ICs before they get to swing on each other.
Because of IC special rules, they are only allowed to attack units they were in Base 2 Base with, and thus can be 'debased' much easier then others.