From an engineering standpoint they have always been interchangeable. Displaying an image is all the same. Displaying bitmapped characters is not that hard either. The issue was what performance you were willing to pay for.

A good computer monitor was thousands of dollars in the early 70s, but so was the computer. As the circuit integration levels increased, the cost of a low end minicomputers and hobbyist computers got down to the point where people would accept working on the relatively low resolution screens that the television standards worked at, typically 250 scan lines on a screen. This is what led to first wave of computers such as TRS-80, AppleII, and later Atari 800 and Commodore 64.

The big turning point was with the development of digital flatscreen TVs and you could show a lot more information. At this point we are getting inside the 20 year rule.

Are you referring specifically to the television standard, or to CRT's in general? The first CRT monitors were on SAGE, but they were vector rather than raster displays.

http://history-computer.com/ModernComputer/Basis/cathode.html