Can you elaborate please on the things that would make you change your rig?
Ok I'll try, I tend not to think about why I make the changes I do, I just do it instinctively I guess.
If I'm getting bites but no takes, or poor hook holds, I'll change to a bolt rig with a long hooklink, because the extra resistance on the line helps set the hook, which helps when fish aren't feeding very confidently.
If I'm struggling to bait up at a distance then I'll shorten the distance between the running lead and the hook, than add a PVA stick, this helps get bait to difficult areas (under overhanging trees etc) and goes a long way to helping eliminate tangles.
If I'm fishing over weed or amongst lillypads, then I may use a braided hooklink for the added abrasion resistance, or use a buoyant bait and small blobs of putty rather than a ledger, which will allow the bait to sit on top of the weed rather than bed into it, this only really works short distance. If I want similar effects but at greater distance I tend to use a helicopter set up.
Method of bite indication makes a difference to the type of set-up I use, When using my quivertip I like to keep it simple and use my "Default" setup for lack of a better word, then tweak it depending on the strength of the tip I'm using, wind conditions etc, but I mainly only change to a slightly heavier lead or feeder.
When using bobbins I like to use a running rig with a fairly heavy lead, because regardless of the way the fish moves off, line will always be pulled so no drop backs, I allow my bobbin to sit on the ground with very slack lines and wait for the fish to lift the bobbin considerably before striking, I also set my bait runner so I can hear line being taken if I get a take when I'm not fully concentrating, but this is only really good to use mid water, as a fish can take line and be snagged up before I've even got to my rod if fishing tight to the margins.
That's kind of all I can think of off the top of my head. As you can tell I tend to use the same old methods with slight variations depending on a certain situation, I hope this has helped Keenook.