1. UV-filtration: There are no definite results of long-time experiments. The technique is useful for short-time application for well-defined situations (i.e. the removal of chemical additives or the destruction of “green water”) but not as a permanent part of a stable aquarium system. The effect depends on the electrical details of the lamp, on the quantity of the water-flow and other parameters. Humine substances may very well be deformed and destroyed; they are chemically not uniformous. In any case UV-filtration a deep technical manipulation of the water-body and the whole living-system (e.g. Fe which plants need for growing will be severely affected by it). For a healthy and stable blackwater tank it is unnecessary and even dangerous.
2. pH-shock: You should be careful in any case, especially if the difference is a full pH-step or more. If you have sensitive and valuable fish (as Parosphromenus or Sphaerichthys) you should adjust the water in no less than one hour, maybe more. Very cautious people do it by a “drip-method”.
On the other hand: I have often adjusted waters for and with Parosphromenus much more quickly up and down, sometimes one value within a quarter of an hour, and there was never a problem. It depends on the individual fish. Healthy and vital fish will mostly stay that (not if the difference is bigger!). But this is not a recommendation. There were situations in which the fish signalled discomfort under the new conditions. In two or three cases I had quickly to lessen the difference.
If you take time, one hour at least, better two, you will be on the safe side, especially with a difference of half a pH-step or less. Remind: Often, the fish are somehow weakened. Then my advice to be cautious holds so much the more.