I sure am glad I just bought 2 new cassettes (one for my rear wheel and another for my CORE) after upgrading from 11 speed mech to 12 speed Di2 a couple weeks back!
Guess I can always pull the trainer cassette off, pick up a cog and save the 2nd cassette for a spare! 
Anyone looking for a like new 12sp 11-34 (105 level) cassette? 