Using the Accusative (Biernik) potrzebować is technically incorrect. The proper way of speaking would be to use the Genitive (Dopełniacz) always. Another way to think about it is that "Potrzebuję tego" is "I'm in need of this" rather than "I need this". That said, this is actually a point of contention with Polish speakers and it is becoming a trend to use biernik instead of dopełniacz in more informal situations.
I think you might have misunderstood what was meant by 'trend'. It's not that it became 'trendy', but it's meant in the sense of 'there is a tendency'.
Acknowlegding the existence of such a tendency says nothing about its magnitude. Therefore, you can't logically conclude that using the genitive case with potrzebować will make you sound weird or old-fashioned. As of 2020, it definitely won't.