In the late 90's they could have specified a bit to signify the 32-bit or larger addressing. A bit of support from say Cisco and Microsoft and you've got the foundation for adoption.
It would only then require relatively small tweaks from application software over the next twenty years and you're in business. As most software doesn't survive unchanged over twenty years, we'd be on IPV4.1 right now.
RIPE and co could make new blocks conditional on rolling out support for IPV4.1. Because there's so much time, that would just be a case of replacing older hardware/software with newer hardware/software as it reached EOL.