Rewigging is a modification of taking the wig off of a doll and/or replacing it with a different wig. Because American Girl dolls have glued-on wigs and the heads are bald underneath, wigs are easily swapped between dolls.
Reasons for Rewigging
The most popular reason for rewigging a doll is to create a new or unique character. With a different wig, a doll can look quite different from her original AG look. AG only has certain hair, eye, and face combinations available, even within the Create Your Own line; those who seek to try other combinations may swap wigs between dolls or use brand new wigs to get new looks. Bitty Baby does not have a wig and one modification is to add a wig to her.
People who purchase secondhand or used dolls quite often purchase dolls who have haircuts, hair damage, or tangled hair that cannot be managed or corrected easily and a new wig starts the doll fresh.
When a doll is sent to the American Girl Doll Hospital for hair damage, the doll is simply given a completely new head. People whose PM dolls have hair damage often wish to keep the original head rather than having it exchanged for a Mattel head, and so, will rewig the doll themselves.
Some people simulate limited edition dolls. For example, since Kailey and Lindsey are hard to find on the secondary market, some people will simulate the dolls by using a similar wig on dolls that resemble them. This is generally acceptable when done to a doll that is then kept in a personal collection; however, selling a doll this way without specifically identifying that it has been "frankendolled" is greatly frowned upon.
Many wigs have side parts; removing and recentering a wig can preserve original hair but change looks.
In standard doll wig sizing, an AG doll wears about a 10-11 size wig. This means that the diameter of their head where the wig edges are placed is 10 to 11 inches. Smaller or larger wigs may be used but may not fit properly.
Bitty Twins wear different sizes.
How to Rewig
If the doll has a wig on, this wig must most likely be removed. If the prior wig is badly damaged, it is not necessary to be careful; if the wig is to be kept, it should removed slowly to preserve it. Once the doll is bald, the new wig may be placed on the head. Many people choose to glue the wig down with a firm adhesive; others who plan to change wigs frequently leave the wig loosely attached so that it can be easily swapped.
A comprehensive guide on rewigging can be seen here: http://www.justmagicdolls.com/articles/rewig/rewig.htm