Petur,
I've answerd this questioin for many people.
If you're going to use some high-tech line like spectra cored double
braid,
make sure you change the sheaves in your mast and foot blocks.
Possibly the sheaves are scored by the old wire and will ruin your new
halyards, and also the shape of the sheave may be originally suited for
wire rope.
A good chandlery should be able to help with sheave selection if you
are not sure.
Good luck,
Pete