Sorry, but the requirements are quite clear and unambiguous.
Everyone affected is entitled to apply for KfW. But what is being discussed here: selling in order to buy again, certainly does not demonstrate intelligence. Not to mention the additional costs that would arise from selling, moving, subletting, buying, and moving again.
Well ...
Sources:
“Your situation applies on the day you signed the purchase contract for your new home or received the building permit.”
.... I have already received the home ownership grant...
“Yes, you can still receive the child-building grant then. The prerequisite is that you have since sold the residential property that was funded with the home ownership grant.”
And some other statements, for example, some have already directly inquired at KfW.
Everyone can judge for themselves in which constellations this makes sense and when not.
IMHO, it is not crazy to think about it and play through scenarios. Especially if you might want to sell the property anyway...