In case anyone doesn’t know, this isn’t really a change of plans. The ISS was intended to run until 2020, and the Obama administration along with our international partners (and the US congress to a degree IIRC) agreed to extend it through 2024. We were actually getting a lot of pressure from the Russians to discontinue it sooner, though that may have been a bargaining tactic to get more money.
The NASA budget simply cannot support manned lunar or Martian exploration programs while still paying for ISS. They would need several billion $ more per year to do both at once.
The other part of the US plan on the table for months now has been to find a way to divest the ISS in the late 2020s. The idea is that if space commerce takes off, some kind of companies might want to manage it or lease/buy it. It’s not clear how that would work finanically, and they’d probably have to figure out how to disconnect non-US assets or buy them from the owning countries to do so. I’m not convinced this will happen.
I’m more hopeful that new commercial space stations will be built in the next decade and will be able to rent space to NASA, universities, and companies needing to perform microgravity research.