Without knowing exactly what you intend to use the loft for it is only possible to offer some general pointers.
That said however it is fairly certain that should you be carrying out a loft conversion to add a habitable room than your existing joists will be insufficient to support the new loadings that would be put on them.
However, if you are only using the loft for light storage and laying chipboard loft boards than your existing joists should be OK.
