Ryanair and Hertz Special Rent-a-car

Hi,

I am interested in renting a car in venice, italy for two weeks in the
period of mid-July. The best combination of flights/car rental fares I
found through Ryanair as well as their affiliate Hertz/Ryanair program.
The two week fare is about 262 pounds (Class B car)

However, it seems that Hertz/Ryanair are having a promotion of a free
car class update in late June (from class B to class D). This offer is
also valid in other European countries besides Italy.

I am tempting to book the ticket now, however, I am wondering if
perhaps in a few weeks the offer will be extended to our booking period
and that I should wait. On the other hand, I am affraid of risking the
decent car rental fare that they are offering.

Does anyone have experience with Ryanair/hertz promotions and if they
are usually extended over and over. In addition, what are the chances
of the rent-a-car fare changing if I wait. My trip is from July 11
through 24th.

Thank you for the info.

Kind regards,