I have a large train station (which happens fo to be the Amsterdam Centraal Station) that's being used at over 300% capacity, and I want to increase its capacity to handle these high passenger volumes, but I don't want to have to demolish the station first. If I just mod the station without demolishing it, will I be able to re-enter my city, or will the change cause a CTD?
I doubt you'd get a CTD. More likely, you'd get a Slider Bug, making your entire mass-transit network useless.
So I would really suggest demolishing & rebuilding the lot in question.
From my experience of making my own Rail/Freight stations it has a chance to CTD. That value you change for capacity seems to have that effect if you don't bulldoze before hand. If it doesn't CTD it will make the lot immortal by default for that old station and may cause the newly plopped one to do the same if used on the same map with the new change. So I would go with xxdita's suggestion and bulldoze before hand before you change any values... Then make the changes and replop.
This insures that you don't get what I ended up with when testing some of my modifications to stations... Also changing properties related to jobs may cause something similar as well. It was not a in-depth test but from what I have of my own work and modding this seems to be the case as well. This is just a heads up. :)
I see. Thanks for your answers! You've saved me a lot of trouble that I otherwise would have created for myself. I'll be sure to demolish the old station first.