Hi, Anja
I recently read an article on this subject that does seem promising. I can't give you a link as I'm 435 posts short of the privilege, but if you want to google it the the title is "Our Experience with 131 Cases of Simultaneous Breast Implant Exhange with Fat (SIEF)" by Masanori Ohashi et. al.
According to the article the breast size after the removal and augmentation procedure decreased by less than one cup size in the six-month follow-up period.
If it helps, the description on DDr. Karl-Georg Heinrich's web site (in Austria) suggests he under "certain specific conditions" uses a similar technique. (I know nothing about him, and just mention the fact as a case in point—while many surgeons do require the implant removal and augmentation to be spaced apart, it can also be done in a single session.)