How are scammers using existing Reddit accounts?

I've seen many posts by multiple users, especially in cryptocurrency related subs shilling links to the same scam. (Wont link it here.) These accounts are often multiple years old (about 4 for many I've seen) and appear to have previously been normal users who posted or commented on various subs over a long period but suddenly all shifted to posting comments (not copypasta but differently worded to fit context in many cases) that all link to the same scam. Are they buying these accounts or hacking them?