Russian banks could go back to using older systems or they could increase the use of their own payment systems.
Russia has its own financial electronic payments system called SFPS. The problem is that SFPS — established in 2014, when the Kremlin feared expulsion from SWIFT for annexing Crimea — has few users and even fewer foreign members. The Russians have been in talks with the Chinese to set up another SWIFT-alternative network, but the project is still at a very early stage. As of February 2020, more than 400 Russian banks had joined SPFS, exceeding the number of Russian banks enrolled in SWIFT. So far, only about a dozen foreign banks are using SFPS, including one Chinese bank, meaning it is really not going to help Russia much on international payment transfers.