Shelling refers to the act of a doll collector using a doll as a “shell” for a specific character. This character can be an original creation, one from a beloved piece of media, an avatar of themselves, etc. The doll is then styled accordingly.
Reshell[]
The act of reshelling is when a doll owner acquires a different doll to replace the previous one they were using to represent their character. Often this happens due to simply finding a doll that looks more like the character they’re putting together, or because they want the doll in a different size. The old version of the character is usually sold, but plenty of people keep old shells to make into new characters (or to just sit pretty on a shelf).