For Acela First Class (or even Northeast Regional Business), always buy an upgraded accommodation up front, from the beginning. From PVD-NYP the accommodation charge is $79 for First Class on the Acela, and $30 for Business on the Regional.
However, if you're deciding to upgrade after already booking a ticket, especially if you're doing it last-minute, a First Class accommodation could end up costing you as much as $152 to upgrade, or as much as $105 to upgrade from Coach to Business on the Regional. That's because you have the pay the current rail fare at the time of the upgrade in addition to the accommodation charge...
Also, be mindful that some discounts require advance purchase, such as AAA or Veteran's Advantage. These both require 3-day minimum advanced purchase, and if you find yourself wanting to upgrade to Business or First Class the day of your trip, you'll be paying the difference between the discounted fare you paid and the current full fare in addition to the accommodation charge.
Always do it up front, from the beginning.
-Nick