Can you make a suit slim? (again)

As long as the shoulders fit, the rest are all adjustable. So yes you could.

42->40 sounds like it should be possible. There is a limit to how much it can be slimmed down though (even if the shoulders fit) but it's not related to whether it has been taken in before or not.

What else are you looking for?  If you want more detailed advice, you may want to either post more details and a picture in the tailors' thread, or be more specific about your questions.

However, slim-fit and cut short tend to go together, and weight loss might in effect make the jacket hang longer on the body. Shorterning (of course) costs extra and can look weird if the reduction is more than trivial.

I actually had a suit jacket altered a bit since I haven't worn the jacket for a long time somehow I too realized I lost a bit of weight and the tailor was quite precise and was able to get the jacket fit me again. He took about an inch off the waist area so it wouldn't look too bulky. But yes it can be done.