Oh gosh...digital versions? I have grave doubts whether this will ultimately be successful. I love ABBA's music( those wonderful harmonies!) but if the digital versions are trying to recreate the image in some way then that creates authenticity issues. But good luck to them.
I feel for you. I recently had to make the call on my 16 year old Siamese. She had survived one lot of cancer two years ago but it had spread to her throat. She didn't know she was ill. She just woke up one morning unable to eat. I rang the vet straight away and she was gone by 1030. Through my copious tears as I held her I knew it was the right decision. You will know when.