I proved this wrong.
My cat was an indoor/outdoor cat for the first 2 or 3 yrs of his life. Now (5 yrs old) he is an indoor cat, if the door is open he won't run out. (Okay so on rare occasions he does. If he is scared by new people or ruckus, and sometimes he's just being mischevious). But for the most of it I can order him back inside.
Now. Because some local councils may require you to keep your cat confined to your property, it is a wise idea to get your cat used to being indoors, before those potential laws come.
If you own your house you are lucky, you can get approval from your local gov and build an enclosure for your cat, this way he may go outdoors still, but confined, the way the local gov wishes. Another option is if you can get permission from your landlord to build a cat enclosure, or more likely, have a large aviary, and fill it with levels, comfort, litter box, some trees (non-toxic to cats) and other outdoorsy things.
For a cat that has been eating, sleeping and roaming the outdoors, I'm going to say now, will be incredibly difficult to make indoors. So in this instance your best option is a cat enclosure or aviary. Of course, if you start introducing her to the indoors now, maybe she'll be 50% indoor by the time they bring in the laws. Or second best option is move to a suburb or town where they haven't brought this confinement law in. : )
Indoor cats will need more enrichment, bring in some non-toxic plants for him or her to smell and play with. Some dangle toys, string, something to chew or bite, scratching post, and most importantly a high spot where he/she can get away from the kids, the barking dog, or the crazy cat loving friend that comes over and traumatises the poor cat.
When you first introduce your cat to The Great Indoors. Even if he/she normally is part indoor. I would recommend you put sheets, sand paper or stop-scratch things onto your furniture, reward kitty when she uses the scratching post, and always redirect her to it when she goes to scratch furniture, carpets, mats, etc. Keep in mind kitty may have been roaming 6 blocks beforehand, and now is confined in one little house. Wouldn't you go crazy too? This means exercise levels go down, so should the calories, you can get an indoor cat food or cut down the amount you are feeding (gradually).
You may also need to get kitty used to the litterbox again, put it in a quiet, low traffic area, (laundry's can be okay.. but the noises are scary to some cats). If kitty won't go when there is only litter in the tray, then place a small amount of dirt from outside in their, so the cat can recognise this as his/her usual toileting spot. Also do not put the litterbox on carpet, cats are messy, it can go everywhere, and boys especially may accidentally go on the outside of the box (sterilised or not).
Remember in all this re-adjusting your kitty. NEVER punish, scream, kick or hurt your kitty. This will only make your cat scared of you, scared to go to the toilet where you might see him/her, etc. So be kind, and always reward for good behaviour (cats are very fond of food, and train the best with food rewards, just use the same biscuits that you'll be feeding kitty for breakfast).
Now as far as my cat goes, I let him outside maybe four times a year, don't do this. Once the law comes in there will be no once a year "Cats can go outdoors" Day. He is always so happy, but it also means for weeks he is eager to get back out there. Guaranteed this is because of the smells and cats that come into our yard and have taken it as their territory.
Some cats are happy on a harness and lead, so this may be an option for having kitty out in the backyard with you, but you may also spend the money and find kitty never gets used to it, so don't get too excited about this option.
How to deal with kitty when he/she is running out the door? Always bring back in, do not allow the cat to stay outside if you can bring him/her inside, do it. Reward kitty once inside. If you have mealtimes instead of a bowl of food out all the time, then kitty is more eager for food, and more likely to come runnning back in if you shake the cat food box/bag/container. I have 250g containers, this covers 2 cats. They are a healthy weight and not losing any more weight. If your cat is fat, do not cut the food down instantly, this is dangerous, take it slowly.
I could ramble on about more tips for keeping an indoor cat all day. But if you haven't had a question covered, feel free to ask, and I'll do my best to give you an answer. : )