Which route?

Not every train has executive class. Which train do you want to take? Only Railjets and Italian Frecce trains have something like it. For Frecce trains I'm not sure you can upgrade. On railjet it's a 15 EUR supplement (above a valid 1st class ticket) . 

On TGV or AVE there are some premium options, usually consisting of a complimentary meal and drinks during the trip. But these are  not bookable for TGV with a pass I think

we’re travelling TGV  Paris to Firenze;

Firenze to Bologna;

Bologna to Siracusa

Siracusa to Rome

Rome to Pars . .  and being into pensionable age . . .are looking for comfort

The frecce from Paris to Milan is not valid with a pass. The TGV on the same route has 1st class but a large supplement. In my view business class on frecce trains in Italy is very comfortable and there’s no need for the Executive class. I’ve done both but not with a pass. I don’t think you can excess up from by a pass. 

You can’t upgrade on Frecciarossa trains to Executive class if you have a pass*. With a 1st class Interrail/Eurail Pass you can use Business class. Reservation compulsory. 

On the route Paris-Milan Frecciarossa trains are not included, you can use TGV services instead. Reservation compulsory. 

*on the not official website Italiarail it was possible some times ago, but Trenitalia (the train company) writes in their terms and conditions that this is not possible. 

For this I highly doubt if a pass is then cost-effective-unless you also plan to do other side trips etc. Just book advance when they open-in IT mostly far later as all anxious people on this forum want to believe. To/fro Sicilia only IC and thus no Frecce and the like (these only run on the newly built hi-speed lines).

IF ´exec´ in actual fact only means an airline style mealplate, you are IMHO always better off by buying the stuff you really like yourself. But there seem to be always very extensive sites that discuss each and every minor aspect of these assumed extra-comfort offers.