Is there an easy way to replace a song I'm testing (with slight changes, like a new tone tone/increasing song volume) , in RS without having to close it everytime? The times I tried it either fails to load, or just hangs while loading the song. 

No this will crash the song if you change a file that Rocksmith already added to the list.

 

One way you could work around that is adding the new version with a different DLC ID (the first of the information field in the toolkit) to aboid any conflict and put the new file into RS DLC folder with a different name so that it doesn't overwrite the CDLC already used by RS (to avoid crashes)

 

Then just go to the shop to re-enumerate the song again and the new version should appear properly.

ah ok. Tought so. Thanks for the advice

Wow this tip is good to test without closing RS each time ! Just increment the DLC ID with a number and GG