NO! You can not go from a large size to a small size. Way I see it, you can fiddle fart around with this yourself and try all the hack plumbing tricks you want to, and it still won't work, so then you can call the plumber. But now, it won't be $500, it'll be $1000 because he will have to undo and tear out most of what you did. Remodeling plumbing can be a real bear alright, you can get to things, working upside down, not sure of what the heck to do next, yada, yada, yada! There are times when a smart DIYer know when to throw in towel and call the pros. It's almost always cheaper in the long run. BTW, I'm a committed DIYer myself, so I'm speaking from personal experience.