A decision by the Russians to possibly shift astronaut training back to a base in the Crimea, now under their control, could lock American astronauts from future Soyuz flights.
Shifting the survival training to Russian-occupied Crimea will require foreign cosmonauts to accept travel there without Ukrainian visas, an explicit acquiescence to the new diplomatic status of the province. Refusal to attend survival training is equivalent to failing the training, which by existing training regulations is an automatic disqualification for flight certification. No Crimea trip, no space trip.
The Russians have not yet made this shift official, so it is possible it will not become a problem. However, the article outlines many reasons why it makes good sense for the Russians to do it.