Not following the previous discussion, but there's a simple answer to the above: Every foot of 3/4" copper (type L, 0.785" ID) is equivalent to a certain length of 1" copper (type L, 1.025" ID). That length is (1.025/0.785)^4.87, where 4.87 is an empirically determined exponent. In other words, one foot of 3/4" type L copper is equivalent to 44" of 1" type L copper.or would there ever be a reason to go from 3/4 copper to 1 inch copper?
That means if you had a situation where you wanted to extend a 3/4" copper line by, say, 44', but the charts or computations say you'll be out of compliance if you extend it more than, say, 20', you can instead extend it with 1" copper and treat that extension as if it were only 12' of 3/4".
So yes, there can be a reason to go larger downstream when making modifications. If starting from scratch, putting the larger pipe upstream where it is supplying more fixtures gives you a greater benefit.