I would upgrade at third base - Lowell's been on a tear, but he's got to fall back to Earth eventually. Although he's on pace to repeat his production from '06 and '07, keep in mind that he was younger back then and was batting in a much more powerful lineup than he is now. Try to sell high while you still can.
Also, I would keep Jayson Werth. He hits behind Ryan Howard and in front of Raul Ibanez in the Phillies order, and you can't have much better lineup protection. He'll be seeing great pitches all year, and if you don't believe me that having a great hitter in front of you helps your numbers, look at Andre Ethier's numbers with and without Manny setting the table for him. I like Werth a lot, this promises to be a career year for him. As for Corey Hart, I'd deal him in a second.
You're right that Lance Berkman has been brutal so far this year, but unless he's injured or something I can't see why he wouldn't turn it around (however long that may take). He has too much power potential to treat him lightly, however. I like your idea of trying to trade him for another struggling first baseman, but unlike most struggling first baseman at the moment, Berkman and Teixeira are really the only two I can think of that have monstrous potential, so it's probably best to hang on. If you're entertaining the idea of trading him for another struggling 1B, try to get Derrek Lee plus another guy for Berkman, although I'd still rather much have Berkman over a hurt, GIDP-hitting D.Lee.
I would trade Aaron Hill while he's hot. He's obviously a great player - his draft position/rookie production/production this year so far all support this, but he's clearly playing over his head, and already his production has started to slowly fall back to more realistic expectations. Looking at the extremely small sample size of previous years' production, he starts the year off hot and then cools down as the season progresses. Also, his numbers are further inflated by the fact that many of the players around him are also playing above their ability, specifically Marco Scutaro (who represents a huge amount of Hill's RBI total). Although it's certainly possible that Marco will stay hot and continue to allow Hill to send him home, it's even more likely that his numbers will start falling in line with his less-than-stellar career averages. I find it highly doubtful that he's finally put it all together at age 33. Long story short, sell Hill high. In fact, you might want to look into packing Berkman and Hill to trade for a good 1B upgrade, and then slot Jayson Werth in at Util.
Also, I can't tell if you meant to say that you wanted to package J.Upton into that trade? I hope you aren't trying to do that, since the kid is going to be huge this season.
Other than the things I mentioned above, I'd let the rest of it stick out and ride. You have a very good team, and if you don't fiddle around with it too much I think it'll produce nicely.
Hope this ramble helped, if you wouldn't mind returning the favor it'd be greatly appreciated viewtopic.php?f=29&t=384532