Full Idea
Because Locke says that personal identity is in consciousness rather than substance, this is a slippery slope which leads others to deny that the self exists (because consciousness is never quite the same).
Gist of Idea
Locke's move from substance to consciousness is a slippery slope
Source
comment on John Locke (Essay Conc Human Understanding (2nd Ed) [1694]) by Joseph Butler - Analogy of Religion App.1
Book Reference
'Personal Identity', ed/tr. Perry,John [University of California 1975], p.102
A Reaction
If you are hoping to have a personal identity that can last for all of eternity, the slightest change now will mean disappearance eventually. There might be boundaries, but then the boundaries would define the identity more than consciousness does.