How hard it is to replace the rca input on the shindo amplifier?

If you're close to Monrovia (just north of Pasadena), take it into Brooks Berdan Ltd. and have the shop's tech Tom do the soldering. He is very experienced with electronics, being a maintenance engineer for a Los Angeles TV station. He routinely works on Jadis, VTL, Audio Research, and other high end lines. I'm sure he'll do it for less than a hundred bucks.

If you use proper tools and apply proper skills to solder a wire, you won't damage plastic chassis of RCA receptacles.
To see how to solder proper you can go YouTube where you will find an arsenal of instructional videos.

In this day and age there is something to be said about fixing something yourself. Heck, I've been guilty of throwing $100 and my time to fix a $20 problem, but it sure was satisfying.

Learning to solder is a gateway to shrink wrap…take this as a warning. 

No need to learn, just see how and repeat.
After 2..3 times of practicing (tops) the job will look nice, shiny and sexy.