@ the "Will it be jarring for the quality to change over such a short amount of weeks?" question, I'd say don't worry about that... like at all LOL I'll give you another perspective to look at it from:
You're worried about your week-to-week current readers seeing a drastic change over the course of a few weeks vs. months or whatever. BUT a new reader who happens upon your series will probably blow through all of that content in a couple sittings max to get caught up. So no matter how long you space your stylistic change out in real time, there will always be some portion of readers who will see it go quickly anyways, so don't sweat the small stuff
Plus, I don't think I've ever read a comic and thought to myself "crap, their art got better already?? I didn't want to see improvement for at least another chapter." The faster the better xD
@ the "how much do you expect an artist to improve/what's your baseline quality for reading" questions, the answer to both is not much, I guess?
To elaborate, art improves in comics naturally, but I don't have like... a standard of improvement I hold a comic to while reading. If/when the art gets better I'm like "awesome :D", but realistically if it's entertaining enough to read from the outset, I'm not going to set it down just because the visuals didn't improve a set amount.
Similarly, I don't really have a quality standard to start reading a comic, art-wise. While it's true that a gorgeous cover might draw my eye in easier, the story content is more important than the art quality usually. An entertaining story can carry poor art, but amazing art can't carry an uninteresting story, imo. I think one aspect to consider is first impressions though, as those are admittedly important! I would make sure that your icon/banner/cover/wev kind of convey your current/best quality style as that is a way to imply to readers where you're going. Like if I see a really pretty banner, and then open up a comic's first page and see less developed art I think "wow they've really improved a lot!" and eagerly charge ahead haha. No clue if that's the case for everyone though. I'll reiterate that story content is a lot more important anyways.