How Exactly Do Tasha’s Subclass Changing Rules Work?

I bought Tasha’s Cauldron of Everything last month and have enjoyed it thoroughly. My question has to deal with the Changing Your Subclass rules found on page 8.

The first paragraph under the title says that characters can change their subclass when they would normally gain a new subclass feature.

The next section, Training Time, goes on about how changing a subclass might take time, money, and/or a quest. How does this work with the previous paragraph’s rule of only changing subclasses when a character gains a new subclass feature? For example, if the character needs time to train or complete a quest, I’m not sure how you could align that with a new subclass feature.

The final section Sudden Change says characters could have an immediate subclass change. Does this mean immediate when a character would gain a new subclass feature or any time at all?

These three sections almost seem like different rules altogether for changing subclasses. I get that the two minor sections, Training Time and Sudden Change, shouldn’t be mixed. However, the rule about only changing a subclass when gaining a new subclass feature is under the main title. This makes me believe this rule should be applied to both of the minor sections. If not, then I feel like this should have been its own minor section as well.

Any thoughts on these rules would be appreciated!

Edit note: I’m fully aware these are optional rules and DnD generally has just guidelines. I’ve been DMing for a number of years now and have that understanding. However, I’m looking for a more rules-based interpretation. Saying that these could be used in any fashion I like doesn’t really help me.