Hey mate. I don't think your bottom plays a part. If anything, I've a gold head bbxb that has been in a dark tank all it's life. It's head is still gold from when I bought him -- in my country, a gold head (GH) is different from a full helmet (FH) - in which you still have dark specks with a GH while an FH is literally fully helmeted when you purchase no matter what size. I would agree with crossing being a matter of genes, conditioning being another factor. I've seen as small as 10" being fully crossed, where others have 5 year old fishes that have reached half of 6th level but never fully cross over. I've seen fish sold as high backs that, after 5 years have fully crossed and look better than fishes bought as full crossbacks by the same owners which after 5 years have not even looked like they are about to cross. Luck of the draw I guess. For water changes, if your area has stable water I don't think it hurts. Personally have done boatloads of water changes i.e. 80% daily for a week after I bought around 1500 tilapia fingerlings for food. The bioload was insane and I did WC because my filter socks literally turn black daily.