Any possible options about house training cats?

I have three cats (all over 6 years of age) who are not house trained and are used to going outside a lot. I’m moving house soon though and will probably not have a garden anymore, so was wondering how possible it is to house-train fully-grown cats? They are much more fond of indoors than outdoors even now, but they usually go outside for the toilet so I’m not sure how easy/difficult it will be to re-train them. If anyone has tips on how to do so, I’d be very grateful.