Only a couple of key locations would be needed. One the UA military can get behind Russian lines, it is game over for Russia in the south.
They did/do have rewards for people turning in Russian military equipment and they do have the I Want to Live hotline for Russian soldiers to surrender.
One problem with a large enough surrender to be effective, would be organising and keeping it a secret long enough. Another would be getting it in an area that would be easily exploited. I assume the most loyal units are in the most exploitable areas.
“For a limited time, if you surrender now, in addition to the Geneva guarantees, each POW will get a daily ration of 50 mL of scotch, 50 mL of bourbon, 500 mL of beer, 50 mL of vodka, and 10 mL of sacred herb.” https://www.youtube.com/watch?v=0RJTlUXU-38 (1:29).