When must I retrofit a circuit?

I'm a DIY'r but a stickler for doing it right. I have a 1980 home that clearly does not meet current code. My question is, at what point of remodeling, do I need to step back and do extra work to bring a circuit up to code? For Example:

I am building a laundry room in an area of my garage. There is a dedicated 20 amp circuit for the designated laundry area but it is not presently GFCI protected. In addition to the present receptacle, I am adding two more on other walls. Am I required to install a GFCI protected receptacle at this time? OK, bad example since it is no big deal to add a GFCI receptacle, but hopefully you get the just of my question.

Another example is the electric dryer outlet is of the older 3 prong type. To upgrade it to the 4 prong type, I will need to run new wire from the outlet all the way back to the breaker which will not be an easy task. Since I am working in the laundry, Am I required to do this upgrade or can I leave the 3 prong outlet in place?

My kitchen is the same way. The dishwasher and range are hardwired and the dishwasher, garbage disposal, and 3 counter outlets are all on the same circuit along with 2 other receptacles in the dining area. The refrigerator outlet is connected to two other outlets on the opposite wall. If I am working in the kitchen doing a remodel, am I required to bring all of these circuits up to current code and add the necessary new circuits?

So in general, what old work can I leave and when am I required to upgrade it to current code?