Pressure Profile In Pipe Flow

Bernoulli's equation is a mechanical energy balance.

v²/2 + gz + P/rho = constant     Notice - no friction loss.

At a pipe reducer, velocity increases/pressure decreases due to Bernoulli and pressure decreases due to friction.  At a pipe expander, velocity decreases/pressure increases due to Bernoulli and pressure decreases due to friction.

You would benefit greatly by doing some practical calculational examples using real pump and fluid flow information.  The pressure changes caused by going up or down one pipe size is usually not that great.  There are exceptions sometimes though, thus you should look at the examples using your conditions to see the effects.

In general, I have found, since a pipe reducer has a somewhat larger pressure decrease caused by two factors, Bernoulli and friction, most engineers account for pipe reducers.  Conversely, since a pipe expander has a pressure increase caused by Bernoulli and a pressure decrease caused by friction, they kinda cancel out, and a lot of engineers do not account for expanders.  Again, it depends.  Work some real examples.

My second thought is, your pipe design pressure should be greater than the maximum operating pressure by a safety factor that is big enough so you don't have to worry about small pressure changes.  And, once again, some calculations by you will guide you on setting a safe design pressure.  Plus, if you consider the greatest pressure / dead head pressure from a pump, there is no flow, so Bernoulli and friction changes are zero.

I've never seen anyone worry about this after they look at some real numbers.  Numbers are hard!