I couldn't resist the temptation and upgraded Kobo H2O to 64GB internal memory. Everything seems to be working fine. So, now both my Kobo Aura HD and H2O have 64GB internal memory. Paradise!
Oh, I forgot to mention how I did it. Although I discovered what is probably the perfect tool for the job (a "hole saw" --- a cylindrical drill bit which can cut a hole), that would mean buying it tomorrow morning. And I can't postpone till tomorrow morning what can be done today. So, I remembered that on one of the Linux conferences they gave me as a gift a Letherman pocket multi-purpose knife and its blade was very sharp and made of very hard steel. So, I used this knife to cut through the plastic on Kobo Aura H2O relatively painlessly. Seriously, it only took me about 5 minutes! If even I can do it, anyone can do it...