When do You Need to Change the Distance of the Jack in Lawn Bowls?

The simplest tactical rule is: change things when you are losing and keep things the same when you are winning. Most game players judge winning or losing by the score, but I think this is simplistic and leads to poorer as often as to better tactical decisions. If we look at any run of three or four consecutive ends in a lawn bowling match, luck as much as anything else may determine the difference in points scored because you do not score anything unless you have the shot bowl. Perhaps a more revealing indication of whether you are bowling better or worse than an opponent is what proportion of the best four bowls in each end of the series are yours! Even if the opponents score shot, if for example you have the second, third, and fourth shots you are doing very well and with any reasonable luck will eventually outscore your opponents.
Shifting to the opposite perspective, just because you have had the shot in each of three consecutive ends, the opposition could have had eight of the twelve closest bowls in those ends. If so, you are being out-bowled and ought to change tactics. I would not want to bet on you  winning the match if things continue as they are going in this situation. As things stand when your luck changes’ those opponents are going to have some multi-point ends. It is good tactics to try to improve your chances now by changing some combination of mat placement and jack length, trying to disrupt your opponents’ good bowling consistency before it leads to a scoring spree.