Bit of an ambiguous question - given we don't know the other dimensions.
However, I find it best to create a new document to the new size, and then import each part individually and resize.
This gives you the option to rethink the layout if say it's a tall advert that needs to be now a wide advert.
Another option would be to use the Page Tool and select the page - in the top bar under Liquid Page Rule - change the setting to Scale.
Input your new size and the layout will scale. You will then need to redefine the items on the page to suit the new size, including fonts, images etc.
Erica Gamet gave a great presentation on this specific topic (creating multiple sizes of an ad in InDesign) to my user group earlier this year. Here are two posts from Erica that explains how to set this up:
Thanks! I indeed didn't mention the new dimensions since I had to change an A4 to 10 different dimensions, so it was more a general question. The way you describe it is indeed what I've been doing, just wondered if there was an easier way to do it. Since it was so time consuming.
Those were very helpful, thank you so much BarbBinder!
Oh good! So glad to be able to help!