Do not reduce the belly - this will lower the bows efficiency and therefore performance.
Reduce the width. Width reduction will leave the bow at the same strain it is at now at the heavier weight.
Width reduction and weight reduction are proportional. You have a bow that is 62# and want to make it 52# that is a reduction of about 16% so you need to remove 16% of the width.
Also removing width is very easy to do and is something that you could easily do yourself. If you remove even amounts along both edges the tiller won't change and you will be good to go.
The correct tools would probably be a fine rasp then a bastard file then finer file and onto sandpaper 60 , 80, 120 and 240 then refinish.