Sounds like an admirable goal. I've never thru-hiked a trail, so I'll leave it to others to give you specific planning advice.
Being a gear junkie ("Hi, I'm Glenn; it's been almost a week since my last purchase..."), you might expect me to tell you to buy all new. But I'm more inclined to tell you to use what you've got, take lots of practice hikes between now and next April, and figure it out as you go.
You'll probably end up selectively replacing gear. If you do, you'll probably want to do so for two reasons: 1) reliabiity or efficiency/convenience, and 2) lighter weight.
Reliability/efficiency/convenience is very subjective. You mention that your tent is old, but pretty tough. Have you had it out in some heavy rain recently? If not, do so - the waterproofing may have deteriorated. The size may be great for the two of you for a weekend, but will you want to live in it for 3 or 4 months? You mention that you really like your MSR stove - by all means, continue to use it; you can probably light the darn thing in your sleep. (You may want to replace those stainless steel pots with titanium, though.) Figure out what no longer works, and replace it.
Going lighter is a two-step process. The first step (which usually results in the biggest weight savings) is to eliminate stuff you have, but don't need. Continuing the kitchen example, if you're carrying two pots with a fry-pan lid, two cups, two bowls, a fork and spoon, a serving spoon, and maybe a cutting board - consider whether you need them. Changing the way you cook (instant oatmeal instead bacon and eggs, freezer-bag supper instead of simmer-stir-mix-and-boil entrees) may let you leave most of that stuff at home, bringing only one pot and a bowl and spoon for each of you (water bottles work just as well filled with tea - no cups.)
One way to check this is, after you return from a trip, unpack everything and put it in two piles: one pile for stuff you actually used (and the "ten essentials") and a second pile for stuff you took but never used. Next trip, don't take the second pile. Do this after each trip, as your technique refines and changes into thru-hiker mode.
The stuff that's in the first pile now becomes eligible for replacement with lighter gear. (This is the fun part.) You can now spend hours going "Oooh, shiny..." while the owner of the local gear shop salivates at the coming sales. When you reach this point, the folks here on the forums (some of whom have thru-hiked the AT) can make specific recommendations as you let us know which piece of gear you're considering replacing and how you'll be using it.
Admittedly, that's only a general overview, but it might give you a starting point for the process. Keep in touch, and don't be afraid to ask questions.
Our long-time Sponsor, BackcountryGear.com - The leading source for ultralite/lightweight outdoor gear:
Affiliate Disclaimer: This forum is an affiliate of BackcountryGear.com, Amazon.com, R.E.I. and others. The product links herein are linked to their sites. If you follow these links to make a purchase, we may get a small commission. This is our only source of support for these forums. Thanks.!