Hey there! I’m not from the Prusa Support but may be able to help.
First of all, do you mean a MK2.5 or a MK2S upgrade? Currently there is no real MK2.5-3 upgrade, as it is basically the same but with 12v instead of 24.
If you mean a MK2S, then the following is for you. I’m assuming you already got the printed parts downloaded, but just in case: https://www.prusa3d.com/prusa-i3-printab.... Make sure you’re using PETG, which are what the Gcode files are made for, but if those don’t work too well then I suggest slicing them yoursel in Slic3r PE. Inside the MK3 3D Printable parts zip file is a file named “Original_Prusa_i3_MK2S_to_MK3_upgrade_Parts_list” that displays all the parts and quantities to be printed.
As for printing singly or in a platter, it depends on how level your bed is. If you’re comfortable with printing lots of different items and your bed shows no sign of tilt (which has a greater effect on larger surface area prints) then go ahead! Otherwise, it’s worth the extra couple of minutes to print one-at-a-time. Also, if you’re printing multiple at a time, keep in mind that PETG can be a little stringy (unless tuned JUST right) and there may be minor artifacts between the prints from stringing.
I hope this helps, and good luck!