Short of IdenTrust’s (offline) root key being compromised (i.e. the “security flaw/short-term fix” case), I don’t see a switch like that happening without significant advance notice. There’s no real SLA, so from a business/legal point of view it wouldn’t make much of a difference (not a bad topic for discussion, but to be honest I haven’t seen much regarding SLAs in the CA industry in general).
It comes down to what’s being promised here and in the documentation. I think a significant change like that would definitely fall into the “advance notice” category. As an example for the commitment on backwards compatibility, an upcoming change in boulder that could break renewal for about 0.1% of all domains Let’s Encrypt has issued certificates for was implemented with an exception list for all affected domains in order to prevent that.