As someone who's done something similar (on the older bikes) I could help out.
I'm around 5' 11" and on my test ride found the M-bar bike a little too 'sit up and beg' and the S-bar bike a little too low.
That's how I came across this site (6 years ago) as I was researching into possibilities.
Long story short.
I ordered an H-Type and immediately removed the standard bars and fitted a low riser bar to give me a grip height somewhere in between an S and an M. (I'm sure I did measure it as some point, but don't know it off-hand).
Here's what my order looked like:
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And here's what I did to it:
View attachment 594193
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IN MY EXPERIENCE:
There's no need to swap stems, but as mentioned above, some curve more than others.
You don't
have to shorten the cables, they don't really interfere with anything, but they will be too long to begin with. In the picture above, I left them alone for about 3-4 months before I got around to shortening them. It's faffy - especially the dog-leg gear change, but all doable for any reasonable home mechanic. What I would say is that your cables will trail even more if you put flat bars on, so it
may be more important for you to do it than it was for me.
What I learned during my research...
If you put flat bars on an H-type it will be around the same height as an S-type from the factory.
If you put flat bars on an M or P type, it will be a very low front end (too low for me). YMMV.
View attachment 594194
The extra height before the hinge on the H-Type stem was useful for me as I was always going to fit bar ends and that meant the bike still folded without any problems.
View attachment 594195
Low-risers have slightly more flex in them than straight bars. Given the Brompton's small wheels and long stem, vibrations travel and amplify so that road buzz on less-than-smooth surfaces is really bad. The risers bars smooth this out to some degree.
I had a set of Ritchey Rizer bars in the shed from another bike, so fitted those with no bother - though they were a fair bit wider than the original Brompton bars. I think I took off about 1.5 cm from either end, but the result was still wider. This suited me as I'm fairly broad shouldered. Someone suggested using a pipe-cutter to get clean cuts, but I just did it with a hacksaw.
I replaced those bars (at 5 years) with another set of Ritchey bars I bought which looked identical, but the curve/rise must have been slightly different as they were almost impossible to fit. I had to really force open the clamp to get them in - and at one point thought I'd messed up and they weren't going to fit. So a word of caution on that.
The brake levers and gear shifters went on easily - as did the bar ends. I could't get the grips of the original bars off without ruining them, but I had no intention of keeping them, so it didn't matter.
Essentially, it's no different to changing the bars on any other bike. You buy something that fits the clamp, and away you go.
Personally, I'd say go for it.
Out of interest, have you ridden any Bromptons to get a feel for what style you'd prefer? Or do you just feel like an H would be too tall? Some people like a sit-up-and-beg riding style.