Question Upgrading the center speaker

Generally speaking your L/R channels are going to be more important than your center, on top of that mixing speaker sizes/brands generally gives worse results than sticking to a similar series.
If youre looking for a large upgrade, consider moving to a new setup with L/R first, then adding in the center/surrounds later if you have to.
 
Having the same speaker for all three front speakers is optimal. That way as sounds move from left to center to right the don't change. Next best thing is to have speakers from a single manufacturer that are designed to be used as a set. Random manufacturer's speakers used as a "set"? No way to predict how the sound quality will be.