thinking of putting new parts on 3/4 year old mountain bike to bring it up to new ones standards,
is this safe?

yup.

nice thanks, wont be doing for a while though, came of bike on friday mashed it up

As long as the parts fit no worries.
Am riding a 6 year old DAWES hybrid with a new Xt Rear mech, xt V's, One Lx lever one Xt lever, a carbon MTB seat post with a racing seat and front wheel is a 20 year old campag racing wheel.
As long as everything is trued and in good condition and fits well you can make your self an excellent set of wheels from old bits.