Yes, you absolutely do want to remove the original library.
Please tell me why? If i do remove it, does my existing sketches with parallel lcd work?
I presume you have solidly soldered a pin header to the LCD which works well in this manner but would be inconvenient when you actually want to more permanently mount the backpack.
Actually i got 3 16x2 lcd:s which i've already soldered a header. And yes, the i2c backpack did have a nice row of headers soldered already. So i went out and test the lcd on a breadboard.
Later i will of course solder the backpack to the lcd.
This brings me a question.
I always used these parallel 16x2 lcd:s with 4-bit mode. So i can save few pins on arduino. Does this i2c backpack require all 16 pins attached on the lcd-backpack or can i use 4-bit mode lcd with the backpack?
LiquidCrystal_I2C lcd(0x27, 2, 1, 0, 4, 5, 6, 7, 3, POSITIVE);
That seems like the i2c is only using 4bit mode? Which is fine for me. Or am i wrong and i need all the 16 pins (yes, the backlight too) to run the i2c backpack correclty?
Thanks alot guys for giving me more information. Just cant wait to get home!
I will be forgetting about 4bit lcd:s and parallel mode. When i get this i2c backpack working ill be ordering more to save pins/wiring!
Just need to get this working first 
Im off from home until end of the week. I wil be then testing more and trying that new liquidcrystal library!