It isn't unusual to have to replace the shower head in a shower. This may be done to upgrade to a shower head that offers different options or to replace a shower head that may have become clogged and is no longer functioning properly. There is a standard thread size for shower heads that makes replacing an existing model with a different model quite simple.
Standard thread size
Shower heads have a standard size and threading that is referred to as One Half Inch National Pipe Thread - Tapered. This is often abbreviated 1/2" NPT. 1/2" NPT fittings will have 5.6 threads per cm, which is 14 threads per inch (TPI).
The male piece of a threaded shower fitting is referred to as MNPT. This is typically the threaded piece at the end of the pipe that comes out of the wall to which the shower head is attached.
The female piece of a threaded shower fitting is referred to as FNPT. This is typically the threaded piece that is part of the actual shower head.