Mixing and slowing changing over is better - less chance of upset belly, and more chance of the cat eating without having finicky problems.
Some great foods that aren't that expensive:
Chicken Soup for the Cat Lover's Soul
Solid Gold (you mentioned)
Taste of the Wild
All these are corn free, and good holistic foods for about $10 per 6 lbs. The best are:
Innova & Wellness (about double in price)
Innova makes a food called EVO - it's about the best food out there, but very rich - not many cats can eat it alone, most folks mix it with another food (foods that rich can upset bellies and cause loose stools). Introduce it slowly (over the course of a few weeks) into their diet.
Wellness makes a food called CORE, also top of the line. Haven't heard any loose stool issues with CORE.
I feed my two the Chicken Soup mixed with the EVO - it's a great mix, and mixing the two makes it affordable (EVO is about $17 per 6.5 lbs). They both have really great bunny soft coats, very high spirits, and are very playful and energetic. I've had cats in the old days on grocery store foods, and there's no comparison - today there's definitely better choices.
Stay away from:
Alot of folks think these are good foods, but in reality they aren't. They are loaded with corn (cats are carnivores - meat eaters, they don't digest corn), and also meat by-products (ie hooves, tails, beaks, snouts, feet, etc. all ground up into mush that they then call meat). Alot of vets recommend SD, but they do so under contractual agreement (Hill's paid for their vet schooling, thus in turn they recommend SD which is made by Hill's).
I'm not much on Nutro either - they make better dog foods than cat foods, and they have so many issues right now, they aren't trustworthy in my book.
Anything found at you grocery store is pure junk, not fit for consumption by any animal - including the high dollar foods in the "organic" section, like Newman's (yes, he makes pet food now too) - they all contain "organic" corn - no corn is good at all!
And stick to dry foods - wet foods are ok as a special treat maybe, but they lack protein and nutrition, not to mention the dental issues (rotting teeth, plaque buildup, etc.).
I'm a cat owner/lover & pet store manager