Yes. I used to work at an investment bank. When the market moved from T+5 to T+3 it was a major project for all internal systems to be adjusted. It wasn't just a matter of changing a SETTLEMENT_DAYS macro in a header file. And even afterwards, it wasn't that, because there were different systems written in different eras and in different languages.
Multiply this by however many thousands of firms, all with their bespoke back-office systems. and it takes time.
Multiply this by however many thousands of firms, all with their bespoke back-office systems. and it takes time.