I'm not entirely sure of how the employment law works in this area. I'm trying to go on a reps course next year to learn more about this in the mean time I'm doing a bit of research with the wonder of Google

I'm trying to cast my mind back to the Ineos Grangemouth dispute a few years ago between Unite and management over the future. Ineos wanted to end collective bargaining, close the final salary pension scheme and reduce salaries to secure the future. Ending collective bargaining is never a good negotiation tactic with unions and early on GTR tried to bring in the OBS grade on individual yearly contracts bypassing the union. Ineos played on peoples fear of losing a job as unlike the railway, the plant can close and production moved elsewhere. GTR can't decide to shut up shop at Barnham and the Coastway routes! At the time Unite seemed to be caught up in some political dispute over Falkirk selections (a reason why I dislike unions been too political) so took their eye off the ball. Ineos offered workers £15,000 one off payments for no union representation, no strikes and reduced salary/conditions. GTR have done things differently, trying to bribe staff with £2000 for signing onto a role that will ultimately disappear in the coming years. Trains will become less safer under DOO and passenger services IMHO will decline.
If people refuse to sign the new contract and get dismissed, I feel that some can claim for either a) No redundancy b) Unfair dismissal. As you mention the guards aren't entirely redundant as they have to operate certain types of traction. It's not a role that should suddenly disappear at the moment given that Southern have conveniently forgotten that it needs to consult ASLEF on extension to the DOO area under collective bargaining.
GTR and the DfT could have ended this dispute months ago if they agreed to the Scotrail solution. It could be argued the union under ACAS came up with a compromise agreement but GTR ignored it. The only winners again are the courts and law firms.