Question: Remodeling a kitchen. I'll be adding two new circuits dedicated to the microwave and fridge. I'll also be reconfiguring the countertops which will result in having to add another outlet to meet code for power distribution.

Haven't done any major wiring changes for a while and I'm now seeing that AFCI is required basically anywhere in a dwelling outside the bathroom. The two new circuits I'm adding will require AFCI breakers so I'm now planning to purchase those.

Since I am adding an outlet down the line to an existing series of countertop receptacles, am I required to also update this circuit to AFCI? I'm not having any luck finding an NEC reference that states this is a requirement, but I also can't find anything that dictates what might be grandfathered in certain circumstances. However, I also know that typically any changes require bringing whatever is being changed up to code so I feel like this is probably the obvious answer to the question.

How about lighting? If I add another light to a circuit serving only luminaires, do I also update that to AFCI?

The existing outlets are already GFCI protected so the only concern is adding AFCI here. Obviously, AFCI is better and I do understand the purposes of the protection it offers but the cost of upgrading half my panel due to a bunch of little changes would not be small.

Edit: I'm located in MI. I couldn't find anything in the MI IRC but I could have missed something.

Answer: 
If you are following 2014 NEC or newer, the answer would be yes, 210.12 specifies AFCI protection where branch circuit wiring is "modified, replaced, or extended". So "adding an outlet down the line" would be an extension of the branch circuit wiring.

The current NEC reference is 210.12(D)(1). I believe it was 210.12(B) in some older versions.
This is really a local jurisdiction issue.

In my city, the rule would be adding an outlet requires a permit which triggers crazy local rules. For AFCI retrofit, my building department would require all circuits servicing the room/area be retrofitted.

In my house, since the lighting circuit for kitchen also feeds family room & laundry room, I would have to fully retrofit those rooms as well. If I had an open floor plan with my kitchen open to the dining room or living room, those would also have to be fully brought to current code.

My AHJ requires AFCI breakers for dishwasher, microwave, fridge, garbage disposal. If the outlet is 5 feet or less above floor, or these items are hardwired, it needs to be AFCI/GCI combo breakers.

AFCI or GFCI outlet controls are not allowed for these items as the local building dept considers access obstructed.

The next town over which is 100 yards from my front door would only require AFCI retrofit for the actual circuits being worked on. They have no additional requirements or amendments to the NEC.

Either call your building department or call a local electrician that works in your city/county. Otherwise it is a lot of conjecture.