What makes you say that, Chuck? I know you're a big fan of the 5100, but what makes you say this would be an "upgrade?"

The typical assortment of reasons, some rational others not so much so... More later...
I may well be mistaken, but don't many consider the 5100 inferior to the 7750? Please feel free to tell me exactly how wrong and foolish I am.
Only the dim, goofy, ignorant and/or certifiably insane... Whoops! sorry but you did say to tell you exactly... =) Really, I am only kidding 7750 lovers out there. It's a fine chronograph movement. I own a number of them myself and I do like the movement. I just like the Lemania 5100 better.

In all seriousness I have some strong reasons for preferring the Lemania 5100 movement even though I am also a big fan of the Valjoux 7750 and also a big collector of them as well...

Some of the reasons I prefer the 5100 include:

  • The 5100 is the only chronograph movement that, because of its construction, can withstand high shocks without the sweep secondhand stopping, as the chronograph functions are powered directly.
    • I'm not 100% certain (I learned this from a 1999 issue of Chrono's Magazine that was quoted in a post here on TZ....) as Lemania has come out with a new movement that is similar to the 5100 but has a more conventional Tri-Compax layout that also may share this feature.
  • It can withstand vertical acceleration of up to 7 g, without the accuracy decreasing noticeably.
  • Its robustness is also legendary, the movement forgives hard blows and has very few claims for servicing. I have heard the 5100 likened to the Soviet T-34 tank or the AK-47 in terms of being able to take abuse and still function properly...
    • Its extraordinary accuracy is consistent over a long period and the intervals between required servicing are long, 4-7 years are usually quoted.
  • It is or at has been used by a large variety of manufacturers including:, Sinn, Bell&Ross, Fortis, Tutima, Alain Silberstein, Paul Picot, Orfina-Porsche Design, Hamilton, Tourneau, Heuer, TAG-Heuer, Revue Thommen among others.
  • It has been in continuous production for over 25 years parts availability is not an issue and likely never will become an issue.
  • Then there is the aesthetics... 
  • The 5100 has more features than the base 7750. You typically see more 5100's with the full Day-Date Treatment than you do with 7750's. You also have a 24-hour military time subdial which you typically only see on 7751 variants of the 7750 (which typically cost more. Even though there is a special 24/GMT version of the 7750 that has been available to firms who wish to encorporate it in their product line for a number of years:
  • I also prefer the layout of the 5100's dial... An Illustration from TZ Classic # 321
  • http://www.timedesign.de/uhrwerke/zifferblatt.html
You'll notice that the main difference between the two is that while the Valjoux uses two of the three sub registers for Chronograph functions the Lemania moves the Chronograph Minute hand to the "SST" on the main pinion. This allows for a much larger and hence more visible and easier to read under duress hand.  
  • Another observation that I have made on my examples is that while Lemania c.321's and c.861's as well as Valjoux 7750 and Valjoux c.72 seem to be susceptible to "Chrono Hour Creep". I have yet to see a Lemania 5100 (or 1342 as used for Omega c.1041 and c.1040 movements) exhibit this behaviour.
That's most of it. I like Valjoux 7750's... Some of my favorite watches are 7750's: Gallet Black PVD, Porsche Design IWC (Both Titanium and Delryn bracelet model's), Heuer Pasadena, and of course the Omega Seamaster Professional Chronograph... But I just really like the 5100's better.
The best car analogy I can find is the Valjoux 7750 and the Lemania 5100 are the Chevy 350 and the Ford 351-C of watches. Both are tried and true designs, nigh bulletproof when treated with a modicum of care, with outstanding parts availabilty and as common as dirt. It is really hard to go wrong in a major way with either one, but some applications are better suited for one over the other. For example, if you wanted to stick either into a Jaguar XKE (just for fun and to shock the hell out of the Cobra Crowd), you'll find the Ford 351c is a much easier conversion because it is about 1-3 inches narrower where there is the tightest clearance around the fuel pump... Because the Jag has such a long snout the frame rails get in the way. However, on a Datsun Z-240/260/280 this isn't a problem, and the Chevy has slightly easier parts availability... I mean when cut I bleed Ford Blue and Silver so I'd want a 351c in either, but I think you get my meaning and analogy...

-- Chuck