The historic agreement between Iran and the P5+1 countries to dismantle sanctions will unlock the floodgates of pent-up demand to enter the Iranian market and secure access to its oil resources.
The key word, however, is "will" as the joint agreement foresees a relaxation of sanctions leading to their final withdrawal spread out over a period of possibly 8 years or more.

Trading with Iran has never been entirely prohibited - just made difficult and discouraged. Which explains why Peugeot announced within 24 hours of the deal being reached that it had entered into agreements to sell cars to Iran and would be looking to establish local production in the near future.
If the legal framework has not yet been changed, what changes will be made to the enforcement policy?
One imagines that any blatant and significant breach would still trigger substantial enforcement risks. Those "greyer" areas where there is uncertainty as to whether there is or is not an infringement would more likely now be left alone. At least, this would be a reasonable expectation as regards the national authorities in the EU.
For the de facto enforcers of the commercial restrictions i.e. the banks and financial institutions, whose approach to enforcement has been strict to the point of excess, the prospects of such early relaxation - or jumping the gun as they might see it - look pretty remote.
Caution may still be the best policy.