Transitioning run of pipe without reducing flow
This is really quite simple, even if vaguely asked. 

As @Daniel Griscom accurately stated in the first comment, "Almost certainly no" (so I'm making this a community wiki answer.)

The question asks if reducing the pipe size (at a particular point, but the particular point does not matter) can be done without reducing flow, and the answer is "no" in basically all cases, though there are some cases where "not enough to matter" would come into play as an answer, but the question does not indicate any information leading to think that is a valid response. In the general case, if there is flow in a pipe, and you reduce the size of the pipe, you increase the total dynamic head and reduce the flow. Period.

More information might be helpful if there was a question about "how much would flow be reduced" but that's not the question.