It is not clear how the interim Geneva agreement between Iran and the P5+1 powers will affect Iran's relationship with Lebanon-based Hezbollah or Hezbollah's regional influence. According to the IAEA's most recent report, Iran's stockpile of medium-enriched uranium has decreased substantially from its prior levels, suggesting that Iran is implementing the Geneva agreement, at least for the time being. One could certainly argue that if Iran continues to comply with the deal and forecloses its nuclear option, it will no longer be able to easily project influence with the threat of nuclear weapons acquisition or a latent nuclear capability. By this logic, Iran may choose to rely more heavily on Hezbollah to make its presence felt throughout the region. This is certainly a concern of other Gulf States, who fear that the nuclear deal does not address the threat that proxy groups may pose to their regimes.
That said, the Iran-Hezbollah relationship may not be so straightforward. Even before the interim nuclear agreement, U.S. and other foreign intelligence estimates assessed that Iran's leaders had not yet made the decision to pursue a nuclear weapon. If this is true, then the regime may not believe that it has to compensate for the loss of a nuclear option with an increased commitment to Hezbollah. Even if the interim agreement is fully implemented, and Iran and the P5+1 reach a comprehensive deal (no small task), the United States and its allies may remove economic sanctions relating to Iran's nuclear program, but will keep in place many more relating to Iran's sponsorship of proxy groups, including Hezbollah.
Additionally, it is important to note the complexity of the relationships in this case. Although it receives aid from Iran and shares many interests, Hezbollah is, to a large extent, an independent organization. And it is possible that groups within Iran, such as the IRGC and Quds Forces could augment support to Hezbollah without the blessing of President Hassan Rouhani and Foreign Minister Javad Zarif, while the latter remain committed to international nuclear negotiations. Where Iran's relationship with Hezbollah is concerned, however, the ongoing civil war in Syria is likely to be far more deterministic than any nuclear deal.