I’m very happy with my 0.4mm nozzle. 0.35 should be quite the same.
You don’t have to change the FW, but you may need some changes in your slicer other than only setting the new nozzle size. First, your max. printable layer height is about 0.8* diameter so your new limit is 0.28mm. You increase the nozzle pressure with the smaller size, that will have an impact on your print quality (direction changes such as edges in your prints) if you keep your print speed the same.
As your line width will also be smaller, your prints will take longer.
I have owned 2 printers over the past 4 years, my first machine came with a .5mm nozzle but I switched to .35mm and have never looked back.
You will have to obviously change the nozzle size in your slicer of choice as well as layer height and speed (slow it down a bit). I print .20mm layer height most of the time at 80mm/s but when I want to do something finer I go to .1mm layer height and print speed of 40mm/s.
Just understand that if you switch every print will take longer from 25% to 70% depending on your settings.
You can go with smaller diameter nozzles. I am currently using .20mm diameter nozzle with ABS and PETG with good results. I am able to print down to .08mm layer height with consistently good results. I have not been able to go to smaller layer heights than .08mm with good results.
ABS produces sharper fine details at .08 than PETG. I get better results from natural ABS than when I’ve tried with colored ABS. With PETG, I am now trying my first colored (MakerGeeks silver) and it is a little less stringy than transparent PETG giving better results.
I’ve been using a .3mm nozzle with ABS at 0.04 layer height consistently the past week with perfect results. Your .2mm should easily cope with it. I have a .2mm arriving tomorrow hopefully to try which I’m looking forward to.
There is another advantage over using a smaller nozzle, any really thin walled parts (1mm and under) will be stronger as more threads of filament are used to build it. I have used mine to print a needle with a 0.5mm x 0.33mm point at 0.04 layer height and it came out exactly as it should. With the .5mm nozzle, it cut off around 2.5mm from the tip, and half of what it did print was just a long single strand of floppy filament - no good.
Swapping them back and forth is no hassle, from cold I manage to heat the nozzle up, remove the filament I last used, cool it down to 165C swap the nozzles, heat it back up, tighten to torque (3.4 nm), reload new filament and purge before my bed has heated up to ABS temps, so if you want to try it then go right ahead.
The only down side is I believe once you’ve removed your nozzle you void warrant on your hot end, which is something worth thinking about. On the plus side, how many of them ever go wrong, and how cheap are they to buy a replacement (£30 I think over here?)