Term for metonymy becoming an accepted word for the original, over time

A metonymy:

... a thing or concept is referred to by the name of something closely associated with that thing or concept

The only example I can think of is tea. Tea can refer to the drink or the evening meal. I think the latter meaning only came about because an evening tea was accompanied by an evening meal and so the term itself became a shortcut for people to say "tea": I'm going to have tea.

It's safe to say that the drink (tea) has become a metonymy for the evening meal (tea).

I'm looking for a term to describe this process, when the original meaning of a word changes into something else, but both forms are used interchangeably?

I think semantic change or diachronic change could fit this definition but I'm not sure. Examples of semantic change from online give examples of words where the original meaning is no longer used and is replaced with its newer form. E.g. in Middle English meat used to refer to all types of food, where as today, meat refers to the animal flesh and food being reserved for all of the categories. Or gay meaning happy but now it's used in its derogatory form.

Solution 1:

This is called a

dead metaphor

A phrase that was formerly a figure of speech, but by semantic shift, its literal meaning becomes what the metaphor applied to.

Another very related example is

breakfast

It's primary literal meaning is a morning meal, but it is easy to see that it was created as a way to express literally 'breaking' a 'fast'. To go further, 'break' here is also a dead metaphor because nothing is being literally broken, that's just a metaphor for a ... let's say... sudden transition.

The idea is that the metaphor really doesn't apply anymore, the meaning of the signs is the new thing and the old meaning is not even considered.

Solution 2:

In your example of "Tea", it is a SYNECDOCHE. Other examples of a synecdoche are when a car is called wheels, and when a credit card is called plastic. Incidentally, I'm from New Zealand, and when we have a hot (roast) dinner at lunchtime, our light evening meal is called tea.