I had mod quite a number of stuffs in my coppermine 1.2.1 version.
Can I still upgrade to version 1.3, while preserving all my changes/mods ??
It depends on the mods. You may find some will be standard in 1.3.0, some you will have to apply to the 1.3 code before they will work again, and some may not work at all.
Without knowing what mods you have installed we can't help more than that.