The iPhone 6s Plus is bigger overall than the iPhone 6 Plus. It’s taller, wider, deeper, and even weighs more than its predecessor—we’re talking .1 or .2 mm on each dimension, so it’s not obviously bigger, which means you might think that your case will fit.
And it may, if you have a silicon or leather case that’s developed some give over time. It’s possible, though, that an iPhone 6 Plus hard-shell case that needs a snug fit won’t really fit on the iPhone 6s Plus.
Bottom line: It’s a case-by-case issue, but there's a good chance your iPhone 6 or iPhone 6 Plus case will fit.