Thanks for your question. What Bastien said makes sense (thanks, Bastien!). To summarize: say that node A proposes value v1 for block 5, but some time after sending out this proposal, A observes consensus on block 5 for a different value v2. Then node A learned the consensus value v2 for block 5 and can move to block 6.
To add to Bastien's answer: node A still wants to propose value v1, despite his value not being agreed on for block 5. A could directly propose value v2 for block 6. However, if some value in the blockchain conflicts with v2, e.g., v1 or some other value contains a different file name for the same metahash that v2 contains, A's proposal will be rejected. How does A learn about it? Because every node is a proposer, learner, and acceptor, A's proposer could first ask A's learner whether v2, the value to be proposed, conflicts with any of the values already in the blockchain. If it conflicts, A shouldn't propose v2 because it'll trivially be rejected by the other acceptors (and should be rejected by A's acceptor as well).
Does this make sense? If not, please write back.