New to premium cat foods

I am the privilaged owner of 2 lovely kitties. One, a 6 year old siamese/dsh and the other a 4 yo dlh. Both are indoor only. Until now, I've only fed them dry food, a pretty decent grocery-store brand. However I am now interested in buying them good healthy food and introducing some wet/canned food into their diet. A few questions, should anyone have the patience to help me out!
a) I've done lots of reading, and the information is intimidating! I understand the value of no by-products, but when it comes to levels, I"m lost. So far I've read that dry food only is too much carbs which can lead to diabetes, that phospherous and protein in too-high amounts contribute to renal or kidney problems, etc. etc. I can't figure out what, then, is the best food? My cats have no standing health issues. Iggy,the DLH, is recently relieved of constipation from a hairball issue. I know he needs something to promote healthy digestion (I also now have him groomed/sheared, which helps A LOT).

b) whats the best way to do the switch over? I know I shouldn't just switch cold turkey. Also, How to switch and introduce some wet food at the same time?

I'm really interested in the Castor & Pollux products (like Organix, indoor, etc) and Pet Guard. I'd heard great things about innova evo, but will all that protein lead to kidney issues? lastly- any recommendations on ordering food on line?
Thanks to anyone who can help!

I do not like Castor & Pollus or Pet Guard. But, that just my opinion.

I order from www.petfooddirect.com sign up your e-mail & they send out % off coupons. Or else www.westcoastpetsupply.com you can get individual cans/pouches from there.

Some basics I look for:
no by-products
no corn
no soy

Since your kitties are young & have no history of health issues, I wouldn't worry too much about various levels. Foods I really like:
Eagle Pack
Wellness
Innova
Merrick
California Natural
Solid Gold
Felidae
Nutro/Nutro MAX
Natural Balance
Premium Edge
Fromms

You have to watch, some foods, like by Nature, the wet is good, dry not so good. Meow Mix cups/pouches are OK(fish heavy), but the dry is crum!

As for the switch, slowly start adding in the new food. Take 2-4 weeks for the switch & it should go smoothly!

I feed my kittens Innova Evo for dry food, and they absolutely love it. The vet says good things about it and everything we've read about it has been good.
For wet food, we were feeding them Avoderm, but they only had three flavors of it, and our cats only liked one of the flavors. We bought out all of our local pet supply store's stock of this flavor, and they weren't replacing it, so we decided to switch to Merrick, since they sell it at a small local business that I like and because we read good things about it.
Their coats weren't especially shiny when we got them from my sister, who feeds her cats store brand dry food or Friskies, and when they went on Avoderm and Innova Evo their coats got nice and shiny, and now that they have switched from Avoderm to Merrick, their coats are even nicer.
My advice to you is to switch the foods gradually if you can (we couldn't, since we couldn't find their old food anywhere, one of our cats got diarrhea for awhile, the other was fine) and gradually mix in more and more of the new food until you're feeding the cat(s) all new food, over the course of a couple weeks.
Good luck and I hope you find a good food for your kitty. (S)he deserves it!

Castor and polluck are GRAINY ... and have some things like fruit THAT ARENT a thing many of us like in a CAT food/// Organix has SOY something that many many cats are allergic to and most of us avoid

Pet guard has some very good wets ... The dry IMHO is not good for the price

White cat lover explained alot of the good ones as will a quick search on here

my kitties (6 mos.) love Merrick's wet and California Natural's Dry and both are very good for them too.

I've also taken to hiding the food and treats in the cat castle and having them hunt for it, so to speak.