So pretty much there will be a slight difference between the XP tournament and the proposed BG idea, that is that lower lvl BGs give more XP for the troops you kill inside then higher level ones, hence for the people doing lvl 80-125 BGs they will get as many points in proportion as those doing lvl 1-20 BGs. Hence it will be difference from XP tournament and a more fair way then current XP tournament.
However you have ruined the strategy behind clearing Battlegrounds, just like some don't wish to clear a BG and still get the points, others also prefer to load up battlegrounds as they build the troops, and then clear then during an event, the second one makes a lot more sense i think, so the changes have absolutely no purpose for helping players to plan, prepare and do.
Now for the rewards aspect we don't know what they will be, but what we do know is that the first tournament will be the most rewards we will ever get from that event. They will implement the change, give a good reward for the event so everyone feels not cheated, even those having to remove some yellows will enjoy the rewards, then when the update is fixed, Nerf the heck out of the rewards just like all other BG events. In a way the help players in the transition i guess... but long term seems another Nerf of an event as they have seen people do too well on XP tournement and BG one, and lets be honest BGs are only worth doing either for the very small early rewards or combined events like many have adapted to.