In this article I will explain what a correction is, what a full re-do is and how one differs from the other.
How lashes grow
A lash in itself is not alive: as soon as its cells form, they die off and compact, forming the hair shaft. The process of lash growth and shedding is constant — just like the hair on your head.
Most masters do extensions using the Japanese technique: one artificial lash is attached to each of your own.
As for a correction, after 2–3 weeks about a third of the extended lashes fall out, some grow out, some change direction. The master removes the grown-out and crooked ones and adds new ones to the empty spots — as a result the lashes differ in length. With each week the look becomes less neat.
About the glue. Any glue has a wear life: the one that is "worn" for a long time starts to lose its properties.
Aesthetics. After a correction the lashes do not look as beautiful as after fresh extensions.
Finances. A correction costs about the same as fresh extensions: the work is no less painstaking.
Hygiene. After a full removal you can thoroughly clean the inter-lash space, where a lot of invisible "debris" gets trapped.
In my opinion the conclusions are obvious: a correction does not save money or time, while a full re-do gives a neater look and clean, tidy lashes. If you have questions — ask!