Is it difficult to change female RCA plugs?

no point wasting your money on stupidly expensive "audiophile" jacks. it will make ZERO difference. You will most likely not be able to find the exact jacks the amp used. The amp most likely used a pair or quad of RCA jacks that directly solder into the board, and then take a screw through the input pannel of the amp for extra support. I recommend getting yourself a set of cheap (but not so cheap that they obviously feel like they're gonna fall apart. no need for gold) *pannel mount* RCA jacks. Radio shack sells a fairly inexpensive set of 4 non-gold, but of good quality pannel mount RCA jacks. These are indiviidual jacks. The way these work is that the jack will go into the hole from the outside of the amp, and then you put a nut around the back end of the jack. I would simply solder some short wires (no need for fancy shielded wire, as there should be less then a cm or two of distance to cover. just keep the wires short) from the original solder holes where the old jack assembly used to be to the replacement jacks.

The only Issue I can think about is that those radio shack jack are not isolated from the metal plate you mount them too. Depended upon the amplifier, this would make zero difference, or might cause a bit of ground loop noise. You can get isolated/insulated jacks that work the same way but have little plastic spacers/washers that go on each side of the mounting panel.