By changing your piping, you changed the available volume of water available...upsetting the balance. So, while you may not have changed the valve, something has changed the balance between your hot and cold supplies. That could be the repipe released some crud that is now stuck in the valve somewhere limiting maximum flow, or there was enough change that you're now getting much more cold, and you can't add enough hot. Note, since a shower head won't flow the max the lines can, limiting flow on say the hot may not mean it dropped enough to notice a change in pressure, but it may have been enough to change the balance. Just like any valve with separate hot/cold valves...the outlet temp is dependent on two things: the actual temperature of the water on each side, and the ratio of hot:cold. If something's limiting how much hot you can get, you'll not get it as hot as you want (assuming the hot supply is actually hot) or the cold is not especially colder than when it was working properly. Where I live, my incoming cold water temperature can vary nearly 50-degrees between summer and winter. An actual thermostatically controlled valve works well for me...I rarely change it, summer/winter, but would have to if it were a more common one that uses manually controlled volume controls for the temperature. Because those changes occur slowly as the seasons change, you probably don't notice it as the day-to-day differences are so slight.