Change 1.75 mm 3D printer to 3.0 mm printer
First it really depends on your printer / extruder. That said generally 1.75 mm is cheaper and much more common.
If one were to change the hotend, likely you will need to replace most or all of the hot end. In the case of my personal hot ends, when I did this conversion I had to replace both the tube and the PETF lined mouth. I did not have to replace the tip, core, or the thermsister.
My advice is to pick a different printer. You see 3 mm on older extruders like J Head direct gear from around 2012-2013 and Bowden style (like the Ultimaker) use 3 mm (actually 2.85 mm).
Possible yes, advised, no.