I don't see that it's worth switching backpacks, but the options you're using seem great to me. I bought a ULA Circuit about halfway along the AT when my old backpack was wearing out and had it mailed to me --- that worked fine and that's still my go-to pack now.
I don't recall specs on the thermarest xlite but wonder if that's enough R-value for a March 15th start. It really depends on the snow year. I started in late Feb in 2010, a high snow year in the south, and used a combination of first generation neo-air plus two GG thinlight pads (one 1/4", one 1/8"). That allowed me to mail home thinlight pads as appropriate, and was warm enough, but just barely on the nights when temps were down into the teens. Similarly a WM 20F rated down bag was just enough combined with down booties and a MB down parka.
There's a big difference in what you need when stopped, in camp, vs. what you need when walking, though this will certainly vary by the individual too. I used a thermawrap vest plus windshirt and that was enough when I was walking. Do prepare for cold wind. Earbags were good for me. I didn't use rain pants, nor want them, but again, metabolisms and "style" vary. I strongly suggest that you get beefy MITTENS, not just gloves. Glove liners are a fine thing in combination with good mittens. Gloves just suck, IMO, when the weather is cold, and for whatever reason I found few to no decent mittens for sale in gear shops along the AT (I lost a mitten along the way and so was looking). Dachsteins if you want the best, PL 400's are good. But something.
I disagree with some earlier comments about pack covers. Where I live, a pack cover is only useful in combination with other stuff, so a poncho works fine. But often on the AT (NOT in the first month or so) it was warm enough that I didn't want any raingear on my upper body, but a lightweight pack cover was nice for the pack itself. Some pack covers are pretty heavy, however; I wouldn't want one of those in any event (and yes, do use a pack liner regardless). But you have a question mark next to pack cover in the second half; if you have a cuben pack cover, keep it for the "second half".
Speaking of "second half", I found that Pearisburg was the right point to swap. I too had an Alpine light parka, and I too swapped to a thermawrap jacket. Worked great. Similarly I swapped to a 32F sleeping bag and was fine with that. Getting into Virginia, out of snow and a whole lot of blowdowns AND then being able to significantly lighten up --- Christmas. Very nice. Once out of the snow my pack was lighter on the AT than anywhere else; less food to carry, less water, pretty warm out so not too much "stuff".
Best of luck! A friend is planning on an early AT start in 2014 and if he heals up and does this, I might be out there hiking the first month or so with him (a month or so of backpacking is my favorite annual weight loss plan ...).