The TOS clauses that the customer agrees to are meant to protect the hosting company as much as possible. Often enough this means that a TOS clause allows the host to just change the TOS at any time, with or without notification (keep in mind that notifications may, due to no fault of the host, not reach their destinations).
But, between that and what happens in practice, there can be differences. The host can try to notify all customers (by email, on forums, on the site etc.), give customers enough time to change hosts, give prorated refunds even if the TOS does not promise such etc. It all comes down to doing the right thing, and trying to maintain the customer's good will, even if he'll end up leaving.