Hello Hakan, the Hohner pro 36 reeds are waxed in. Since these reeds were installed a long long time ago, the wax has crystallized so you will need to chip a little bit of the old wax out and pry it up with a small flat screw driver. Once it is removed, carefully remove all loose wax with a razor knife. Set the new reed in to see if it sits flat with no obstructions. If this is good you will need to wax the new reed in. You can purchase accordion wax on EBay, make sure it is accordion and not just bees wax. I have purchased a small brick For about $20 USD. You only need a small amount less than a cap full. You will need a small thin disposable fine brush. In a container like a tuna can, heat the wax until it starts to smoke a little. The wax is ready to apply. Cover the other reeds next to the one you will be replacing to avoid dropping wax on them. Blow all loose wax and brush a layer of hot wax in the seat of the reed, carefully set the reed in and gently press down. Do not over fill with wax that will spread. Now that the reed is set brush some wax around the reed avoiding the tongue of the reed. If this happens you will need to remove the reed and remove all the wax and restart the process. Let the reed set for about 20 minutes so the wax can cure a bit and you are ready to go.
Hope this helps
Yes you will need to add or it will leak, it is simple to do, just take your time and cover the other reeds so they do not get wax on them. Also check your gasket to make sure it is not broken while you have it taken apart.
Hakan, I should have clarified. Yes on the reeds that you replaced. Check and see if there is loose broken wax on any of the existing reeds you are not replacing, if not, no need to wax existing reeds. Once you have closed the Melodica, play and if you hear that a reed is weak mark that reed and check to see if you need to wax but most likely not.