The AR-15 is one of the most versatile gun platforms on the market, which may explain its popularity. It can easily be configured for anything, from a close-quarters combat weapon for home defense to a rifle for hunting.
Changing the gun from one purpose to another merely requires changing the barrel and optics, leaving the functioning of the gun alone.
While few people actually repurpose their AR-15s by changing the barrel length, many customize their gun, making it fit the use that they have for it. While gunsmiths are regularly employed for these changes, there isn’t a single change on an AR-15 that can’t be done by the owner, assuming they are fairly proficient with tools and have the right ones.
There are a few specialty tools needed for some types of modifications, especially for changing out the barrel. However, these tools are readily available for a reasonable price. If you like working on your own guns, it’s well worth the expense of buying these tools.