I do not think it will work at all. Usually,it works the other way around. If you have a FLAC code in 7.0, it can easily work in 8.0 but definitely not the other way around. If it works, the very purpose of bringing a new version by the developers is defeated. Prof. Gali Madhavi Latha might be able to assist you on this. Check out for her profile in Researchgate.
Plastic Hardening Model is newly introduced in FLAC 8 based on the following work:
Schanz, T., Vermeer, P.A., Bonnier, P.G., 1999. The hardening soil model: formulation and verification, Beyond 2000 in Computational Geotechnics. Balkema, Rotterdam
This model is version specific and does not work in Flac 7.0. However, you can explicitly incorporate this model into FLAC by writing your own code. This is a huge task and someone has to really work hard.