Modding watches is not relatively new, but it is a good way for people to put their own spin into their watch. However, watch modding cannot necessarily be done by yourself unless you are willing to pull the watch apart and risk it breaking.
Watch modding can be as complex as you want it to be, as anything on the watch can be changed. From the hands of the watch, to the dial itself as well as the crystal that is used. Some people have also gone as far as to replace the entire movement.
However, watch modding should not be done unless you are experienced and know what you are doing as it comes with the most risk. When it comes to changing straps on a watch this can be done by the person themselves as it does not require much effort and can be done very easily.
I would recommend to find a reputable watch modder who has shown their previous work as this gives you the confidence knowing that they can change the watch to what you want. I would be careful when purchasing watches online to make sure that the watches have not been modded. This can be done either by changing the movement to maybe a cheaper movement but still listing it at its original price.
I would generally advise to check the movement and the reference number of the watch to be able to cross-check with other listings of that watch to ensure that the watch you are buying has not been changed.
For watch modding the most common brand has become Seiko. Probably because of the cheapness of parts that are available and how easily parts can be sourced for the watch. Although people do mod watches such as Rolex it is not as common when compared to Seiko.
An example of a Rolex modded shown below.
The original Rolex Explorer II 
The watch modded after
Example of a Seiko modded shown below.
The original Seiko SKX007 
A Seiko SKX007 with the batman mod
If you are planning to mod your watch, it is recommended to buy a broken or very cheap watch. This is so you can get used to changing or taking apart a watch and seeing for yourself how complicated it could be. It can also be a new passion for people to be able to mod their own watches, however please do it at your own risk.
The Watchlist 