ARFC2 Migration

Hi Experts,

                  Recently we upgraded to Netweaver7.4. We are using NWDS 7.3 now. I am trying to modify existing application which was developed in NW 7.0. when I click on Model I am seeing an option called "Migrate to ARFC2 Model".I have two doubts here. Is this migration to ARFC2 complusory in NWDS 7.3? Also I want to reimport model due to changes in RFC. If migrate to ARFC2 is compulsory should i do it after re-import RFC or before re-importing? Any Guidance can be helpful for me.

thanks,

Nooruddin