Sylvia Plath's poem "The Mirror" is an example of a personification poem. Personification is where non-human and non-living things are given the qualities and characteristics of human beings. In regards to the poem, the mirror is the speaker. First of all, mirror cannot speak, although humans can.
The movement of the personified mirror to being a lake signifies the change which takes place in the woman seen in the mirror. At the opening of the poem, the mirror recalls the reflection of the wall across from it. No one is described as peering into the mirror.
The second stanza, on the other hand, depicts a woman peering into a lake (metaphorical for the mirror). Like the mirror, the lake reflects back the image of those who peer into it. For the lake, the image of the old woman who has peered into it over time has changed. No longer a young girl, her youth swallowed by the lake, the old woman breaks the surface of the lake trying to escape its reality, like a fish gulping air.
In me she has drowned a young girl, and in me an old woman
Rises toward her day after day, like a terrible fish.
The change from the mirror to the lake could symbolize the differences between a lake and a mirror. A mirror is stagnant. A mirror does not change. When one looks into a mirror, they see the same image time and time again (outside of cosmetic changes and aging). A lake, on the other hand, is not stagnant. Ripples can form on the surface of the water-distorting the reflections seen upon its surface. Some who look into the lake's reflection may come to appreciate the ripples (given it shows them something different that who they actually are). Others may look into a rippled lake in order to distort their image in attempts to "trick" themselves into thinking they are something they are not. Unlike the lake, mirrors simply cannot be tricked.
All said, the mirror could simply have been changed to that of a mirror because of their similarities. On the other hand, this change could be because of something deeper-like the lake itself.