Indoor to outdoor, any advice?

I have 3 cats all from a local shelter. The youngest, a 2 year old male, feral when I adopted him, but the soppiest cat you could ever meet now, and 2 little old ladies of 16-17 years old.
The problem I have is that having moved house 6 months ago, I vowed I would not have a litter box in the house as all 3 cats are able to go outside, and enjoy their freedom. On moving into my property I realised a catflap would be almost impossible to fit, and so out came the litter box.
The cats always use it and have never had an accident. I finally found a way to fit a cat flap and was determined that the litter box would be removed- it wasn't, I was concerned that the old ladies would need it and so it stayed and of course the old ladies now refuse to go outside, and if they go out in the garden, they come in to use the litter box!! Silly girls!
Things have changed in my house and the litter box must now go. How do I make the change? At the moment I have put the litter box outside by the cat flap and blocked where it used to be in the house. I put the cats in the litter box every hour or so in the hope they will realise that is where they must do their business.
Any other advice to aid this transition will be gratefully received.

Hi,
My three cats are indoor outdoor cats and I still have two litter boxes. They donâ€[emoji]8482[/emoji]t use them much when the weather is dry, but they donâ€[emoji]8482[/emoji]t like getting their paws muddy! I have covered litter boxes, I buy cheap litter and throw it away and wash the box, I canâ€[emoji]8482[/emoji]t be bothered to scoop, and I do one box a day. It seems a bit hard to expect your elderly ladies to change their habits.
I also only allow mine out in the day time, and it would a long time for them go all night without spending a penny.
Anne

I agree that you should keep a litter tray indoors even if they go outside, especially for your older cats. As mentioned above - they may need it overnight. Having them use a litter tray instead of outside has it's advantages - you can keep an eye on what is being produced (so can detect ill health sooner) and also know that they aren't using someone else's garden as a loo! I think you're just going to have to find somewhere to put it - why does it have to go?

Is the change pregnancy??

They should work it out if you take away the litter tray I have a cat who goes out when I am back in London but stays in when I am in brighton and copes fine.