Buenas días, Oliver,
unfortunately my Spanish is still too poor, that's why I respond in English. I hope this is fine with you.
I think it is quite normal that you face changes or new requirements during realization, as nobody is able to foresee and cover 100% of the needs during blueprinting (that's why you should establish good OCM and Change Requirements Management processes). Then you cross-check against what has been agreed upon and signed off in the BBP and change/amend accordingly.
ASAP8 is not restrictive about that and gives you the flexibility you need: you follow your project approach, e.g. using the ASAP8 phases, tasks and deliverables and consider the updated requirements accordingly. Yes, you might go a step back, e.g. change customizing or re-write test cases, however this is also quite normal (so hopefully an exception, not the normal routine -but this is another story).
Best, Tilman