A good argument can be made for re-working a load any time a component is changed. The only way you can know whether the resultant change of a given load after doing so will be small or large in your setup is through experience. After a component has been changed, it is not uncommon that when the previous (original) velocity is restored, the load will shoot much like it did before, including optimal seating depth. However, that is not a guarantee...one can only test to determine whether restoring the original velocity works or not. Without that, there is no way to know with any certainty.
For some of the variables we optimize during the reloading process, the whole purpose for going through the process is to identify the optimal "window". In order to identify an optimal "window", you have to define BOTH edges of the window. That is the only way you can be 100% certain that you're loading to the middle of the window, thereby enjoying the benefit of maximal resistance to the load changing and going out of tune under different conditions. The potential problem with only doing minor tweaks after changing a major component such as brass is that you might find a load that appears to shoot pretty well, but you really have no idea where the load falls in terms of the optimal window because the abbreviated testing process may not be enough to define both boundaries of an optimal window. If the tweaked load is right at the edge of the optimal window, it could easily go out of tune later if the conditions change even by a fairly small amount. However, re-doing a full testing procedure consumes more time, reloading components, and barrel life, and is therefore a lot more painful than a minor "best guess" tweak, but offers more certainty in return.
I have had loads with a new Lot # of brass or bullets tune in at exactly the same spot in terms of both charge weight and seating depth as the previous load. I've also had them differ markedly, especially if the internal volume of the new brass or external dimensions of the new bullets were quite different. Even with that information in hand (i.e. new internal case volume, new bullet dimensions, etc.), the best you're likely come up with is an educated guess, which will then have to be tested. So in terms of the amount of change that might be necessary, the honest answer is, "as much as it takes". You might get lucky and only need one tenth grain or two powder adjustment, or a few thousandths seating depth adjustment to bring the load back in in the new brass, but again, no one can tell you that with absolute certainty. My advice is to do as much testing as you can stand (or afford) after changing components. In the long run, you will usually be happy that you did.
