I did this to the J/30 that I bought in May. The halyards were ancient and were wire/rope halyards. I replaced them with all rope. The original purpose of wire/rope halyards was to minimize the stretch that was inherent in older, lower-tech line. Many newer lines have less stretch, are lighter, and are vastly easier on the hands.
Before you put up a halyard, it is critical to inspect your sheaves. Sheaves are usually cheap and easy to replace, but if you skip the step, you can destroy a new halyard, a lesson I learned the hard way when I rolled the dice on the main halyard sheaves on my boat. I only have the main halyard that goes to the masthead and I didn't want to go up on a single, old halyard. If you're curious, a write up of that experience is here: http://www.rambunctiousracing.com/sheaveissues.html
If the sheaves have a deep groove that is meant just for wire, then they'll need to be replaced. Many can accommodate either wire or rope. If the latter applies, examine the sheave for any burrs or wear spots that can abrade the line; it doesn't take much. See photo below for an example of wear spots.
As far as line goes, if you are just doing recreational lake sailing, then you do not need high tech line, tapered halyards, or anything else that will make the replacement unnecessarily expensive. I would suggest a decent quality line, probably with an 8mm diameter, that is easy on the hands. StaSet X is a good choice. Have the vendor install a shackle on an eye splice and a reeving splice if you aren't comfortable with splicing your own halyards (I wasn't).
The reeving splice is just a loop made from the line's cover that makes running the new halyards easier. I just cut the old halyard above the crimp and secured it through the reeving line and along the new halyard with rigging tape. Then, simply pull the old halyard out backwards and the new halyard will feed right through.
Good luck. New halyards are very nice to use and really make it look like you care about the boat. By the way, I ordered all my running rigging from Layline and they did an excellent job.
Below is an example of wear spots on a sheave. The spots are on both sides on the sheave roughly near my thumb