What happens with the world records that were scored before the rule change?

They stand. But if you look at the current records (based on https://en.wikipedia.org/wiki/List_of_world_records_in_swimm... ), most of them were set after the 2009 ban. In other words, the full-body suit advantage was not overwhelming.

While the advancement to carbon fiber in running shoes is similar, the exploitation in swimming was far more extreme. This was an unintended side effect of the suit when shifting from skimpy Teflon Speedos ("Fast Skin") to full-body polyethylene LZR suits. The key issue was layering multiple suits. Wearing two suits would trap a huge air bubble between the layers, pushing your body above the surface and reducing your drag substantially.

The multiple suits issue is generally downplayed because it doesn't promote Speedo's new technology and casts swimmers as cheaters. Imagine showing up to a race with your new carbon fiber running shoes, only to see your opponent wearing "Moon Shoes" strapped over their high-tech shoes. It looks ridiculous, but there was no rule against it at the time.