Actually the commission does not need approval from either of the candidates to change the rules. They can act unilaterally and the candidates can either accept or boycott the debate.
And they went ahead and made a unilateral decision which gives Biden exactly what he wanted and which gives Trump absolutely nothing. Blatant favoritism. Just as with their choices of moderators.
If that is the case then Trump should have a rally in the biggest venue possible. Believe a lot of people would tune in to see what and he can lay everything out point by point
So, the commission is giving Biden cover and a way out.