If you haven't already changed your ticket, I would recommend not doing so. If you are retired, and you are already flying into Punta Cana, what I would do would be to change any hotel arrangements made in Sosua to give time to do some touristing in Punta Cana, La Romana and Santo Domingo prior to heading to Sosua. What's your rush?
There are many small hotels where you can stay for economical rates. If you need suggestions for economical hotels, see the
http://dr1.com/directories/Hotels-Resorts.html and check by price range.
Using the local intra-city bus system is a wonderful way to see the country for cheap if you have time. And from what you tell us, I don't see what the rush is to get to Sosua. I think it would be easier to change your lodging reservation rather than the airplane ticket.
I would stay in several hotels in Punta Cana, so as to get the know the area -- it is a big beach strip (Cap Cana, Puntacana, Veron, Cabeza de Toro, Bavaro, El Cortecito, Macao). I would take public transport to get from one place to another. The buses are cheap. Or maybe rent a car and head to Dominicus and enjoy the beach there, move on to Bayahibe and enjoy the beach there. See La Romana. Turn the car back in and take the bus to Santo Domingo. Spend three days there. Check out the other tourist stuff. A week's time. And then take the Metro/Caribe Tours bus to Sosua. Would be a waste to be east and not see the sights.
Just make sure you have your smart phone activated with a local plan so you can use it frequently. And avoid driving at night! That's why am recommending you to stay in several hotels so you can move forward and not spend your time going back and forth.
