I'm about to replace the drivetrain and transmission on my old Bianchi with something newer and am leaning towards the 9-speed Shimano Acera groupset (https://www.amazon.com/gp/product/B00...A1LAO3E1F13N8P). Mainly because its reasonably priced and better than the 20 year old 7-speed Altus system I'm currently running. In addition to this, I want to change out my 26" wheels for 700c for better street/commuting purposes since that is my primary riding style. I'm leaning towards a Sun CR18 wheelset with Shimano M430 hubs and 14g spokes. Would there be anything that would keep those wheels from working with my frame and fork?
Also back to the topic question, I also want to change my handlebars. Either to bullhorns or butterfly/trekking bars. Would rather have bullhorns, but will the shifters in that groupset (Shimano Acera SL-M390) work with that type of handlebar? Any suggestions on different types of handlebars or handlebar setups?

Does your brake pad fit with 700c?

I have trekking bars , they work with a gripshifter . & my Magura Hydraulic rim brakes ..

Dont be daft..
stick with the wheel size the bike was built with , N+1, go shopping next for a 700c bike if thats what you want.

Only way to change wheel sizes and have brakes is Drum brake Hubs
since that is not a disc brake frame ..

"stick with the wheel size the bike was built with"
+100, just get yourself some nice slicks like Michelin City Tires, inflated to sane pressures. They are quite puncture resistant and will help cushion your ride over potholes, pavement joints and drain grates you are likely to encounter on your commute.

As fietsbob says, if you want a road bike, buy one; you will not be able to make your mountain bike into one.

The idea was not to make it a road bike, just better as a commuter. After more research, I agree the new tire idea is better than wheels. So suggestions for handlebar setups with trigger shifters? Also good idea go from 7 to 9 speed cassette? The drive train is as old as the bike and I don't want to replace the entire bike yet; I'm pretty attached to it.

width of rear triangle may be an issue you will have to deal with. 7 speed hub OLD is 126mm. 9 speed MTB hub OLD is 135mm. You will need a new rear wheel as a matter of course, unless you have that really rare 7 speed freehub/cassette instead of a threaded on freewheel. Then, you will have to cold set the rear triangle, making sure that you realign the dropouts so they are parallel after spreading the stays. Good luck with that. It can be done, but it isn't quick or easy to do it correctly, nor is it cheap to replace eevrything you will need to replace to make it work. A new bike may be the better use of funds.

The width of the rear triangle is 135mm, I'm just not sure if the hub will accept 9-speed. And it is a cassette, not a freewheel.

OK, looking at the picture of the bike, it appears to have a ton of seatpost showing as well as a ton of stem. This would point to a bike that is possibly poorly sized for you. If this is the case, a new (used) bike would be a far better idea.

The bike fits me well by mountain bike fittings. I have it adjusted that way to fit my style of riding, which is primarily commuting. A new bike is in the plans, but it won't be able to happen anytime soon. Which is why I'm working on making this one a better commuter for the time being. But yes, I plan on getting a road bike when funds become available. So...eventually..

The groupset you linked is over $200. Adding bars, tires, etc. will get you up near $300 to "commuterize" your current bike till you get one better suited toward commuting. Seems like putting the $300 "upgrade" money toward a bike that is better suited to your needs would be a better use of the cash.

As the OP reported above, the dropout spacing is already 135 mm and that wasn't unusual even for 7-speed MTB bikes. My '92 Trek 7000 was 7-speed and spaced 135 mm. Also, 7-speed cassette hubs were VERY common. The Trek 7000 I mentioned, a '92 Trek 1420 road bike, my son's '96 Trek 1200 and many other bikes of the same vintage all came with 7-speed freehubs.

Note to the OP: Your dropout spacing is good but your 7-speed freehub will not take an 8, 9 or 10-speed cassette. You will have to change the freehub body for a suitable one and redish the rim or buy a new properly fitted wheel.

+1000 on don't even THINK about changing the wheel size, just fit more suitable tires.

I know you didn't mention them, but I'm loving the sweep of my Jones loop h-bar, On-One Mary, and the Clarence bar are among others with some sweep back to consider maybe, and for less $$. I'd never ridden with swept bars on a bicycle before and I really like it.

I'm in the "change tires to fat slicks" camp too and if the group set you have still works, just pull it apart and give it a good cleaning and keep running it. Maybe some new cables if things don't feel so slick anymore.

Yeah, I keep doing more research and decided just to change the tires. Also just to strip it down and clean everything up then run new cables, new chain, etc. Putting my money towards a new frame instead, and will start from there with my new bike. Thanks for all the input!

I'd suggest you size the new housing at the rear derailleur to a bit more reasonable length, and if your arms are getting tired raise the nose of the saddle. The idea that one can make a fantastic bike by starting with a frame is a fantasy, as well as a time vacuum and money pit. It's most often better to get a stock bike that is close to your requirements and then change out the parts that don't quite meet your needs. By the time you sell the spare parts you're way ahead of a full build. If you just want to be able to say "I built it from scratch," then have at it.