Going to a lighter mast.
 

When using a lighter mast than was originally in the boat, what can be expected in the boats performance? Also is it possible that some free ballast may have to be removed or moved? This is based on a 45' wooden cruising ketch.

It depends
 

Hello,
First, I'm assuming that the new mast's dimensions are based on thorough, conservative calculations -- basically, that the old mast was too heavy, and you are correcting things. If so, you can expect less heel in a given windspeed, and thus less weather helm, and less frequent reefing, the degree of which will be determined by how dramatic the weight reduction is.
If it is a very large reduction, you might improve acceleration. Vessel motion will likely change, and in extremes this can lead to a "snappy" motion, but then we are talking about carbon fiber and PBO.
Here's an exercise that might illuminate things for you: determine what your current mast weighs, and subtract from that number the weight of the new mast. Let's say the difference is 50 pounds. Haul a 50 pound weight halfway up the current mast while you are under way, and watch what happens.
As for ballast, lightening the mast will lower the center of gravity, so technically you could remove an appropriate amount of ballast and have the same righting moment, but that is probably not a good idea here. You are more likely to shift ballast, as the vessel will be less bow down in a given windspeed.
Fair leads,
Brion Toss

In general a lighter mast is better but there can be interesting exceptions. I have the big Brewer catboat, a Chappaquiddick 25, with a monsterously heavy solid wood mast. Marmalade is wood and I've sailed against a glass Chappy 25. Both boats have been handled by the same yard in the past and it's known that the displacement and general weight distribution of both boats are quite similar. The other has an aluminum mast. In many conditions Marmalade romps past her sister. We finally figured that it was not my sailing, but rather the very strong form stability of these boats. The other responded to any level of cross sea more rapidly while Marmalade's massive stick made for a longer moment and didn't spill the air out of the sails so badly.

Curious
 

I suppose I'll really find out next season. The boat is double planked and fairly heavy, so I don't know how much of a difference my momentum will change. Hope fully not too much. As far as rolling goes, we'll see. I can always play with the free ballast. I'll probably post results here and on wbf. I suspect the lighter the boat the greater the change.