Hi all, I am the new owner of the Merlin 3271 (Penelope, a beautifull Summer Wine design). I sail her in Luc sur Mer (Normandy, France) for two weeks.

After some outings in very light winds I am planning to make the conversion to a new rules spinnaker. What kind of changes would I have to do on the rig? How much would it cost? (secondhand stuff considered)?

Thanks for helping.

would suggest twin poles (long) and a new or 2nd hand spinnaker plus bits and piesces

The halyard block on the mast can stay where it is?

Halyard block can stay where it is, yes.

The point at which your pole uphauls connect to the mast probably needs to be higher than it currently is. I think they are usually fitted just below the jib halyard sheave. This means the longer pole sits at a better height when stowed along the boom.

Your new pole(s) need to be 2.3m - measured between outboard ends.

OK, I understand. Thank you for your help. 

What is the surface difference between an old spinnaker and a new one? Does it really make a difference when sailing?

New spinnaker is 10m2, the old was about 8m2 and not such a good shape, the rules limited the length of the foot and leeches of the sail.  There is a very big difference using the new larger spinnaker.