I am assuming you are using FTM.
With the BROTHER selected in the tree.
Instead of 'Edit' go to 'Person' (in same row) then select 'Attach/Detach Person' then 'Detach Selected Person' then select the Father and Mother you wish to disassociate and click 'OK' then go back to 'Person', 'Attach Mother' you will be given a list of people in your tree, select the appropriate person and click 'Attach' then 'OK'. et voila the change should be accomplished. No one will be deleted. I am assuming the father is unknown, if he is known one can add him in the normal way if he is new to your tree or use the attach method if he is already in your tree.
I can't check the procedure on Ancestry on-line as I don't currently have a tree on-line.