Knockback is Energy's damage mitigation/crowd control feature. You will gimp yourself if you convert all your KB to knockdown, because KD is less effective than KB. That said, KB can be frustrating to team mates if not used properly. So the key here is learning to play (L2P) the build correctly, in a way that benefits you and your team (git gud).
1. Think before you blast. What is your position? What is the mob's position? What is the tank's position? Are the mobs locked down yet? Where is the target-based debuff active? What is my backstop: where will the mobs go when (not if) I knock them back?
2. Hover. Hover should be required on all Energy blasters, period. Hover above the crowd and KB down.
3. Focus on the outliers and runners: all groups of mobs have them. The melees don't want to chase them down anyway, so make them your b*tches. Hey look: that oddball is targeting our healer or controller -- take it out and be the team's hero, instead of its goat.
4. Slot KB2KD in your AoEs, not your single-target attacks: Energy Torrent, Explosive Blast, and Nova. That will preserve the primary utility of KB in the Energy set, and save you some cash. I'm usually a ST kind of player, so I don't always train ET or EB, meaning that I need only 1 KB2KD, in Nova. YMMV.