<<Can an ordinary battened Main be converted to full batten?>>
Yes, but your sailmaker should advise you it will not be cheap and you might like to consider using your part battened main as a cruising sail and reserve your new fully battened main for racing only.
<<Is it worth it?>>
If you are after line honours or need to improve your speed a bit you have to consider the cost, the shorter life and extra repairs needed for a fully battened main.
If you are racing on a handicap system your part battened main will taken into account, so what will be the real gain?
Racing or cruising - the most efficient sail will get you there quicker - and a fully battened main holds a more efficient shape.
It will also keep it's shape longer - saving money in the long term.
It will also be easier to drop into lazy jacks - saving hassels from day one.
IMHO it's no contest
I've bought a new mainsail this season (delivery this week hopefully!), and initially considered a fully-battened main. I went for a 2/3 battened main instead (because of hardware cost), but while researching I was led to believe by a Sailmaker (dont recall which one) that upgrading to a fully battened main later would be possible, and I wasnt advised that it would be a big deal. so, it's probably worth calling around a few sailmakers to check out the cost....
No it isn't worth it IMO. There are a lot of advantages to a fully battened main and I would disagree that it implies higher repair costs, in fact I would say the opposite as one of the prime repair jobs on a normal main is batten pocket inner end damage. A FB main will set better, keep its shape for longer, set well in light airs (when a conventional main will have the air shaken out of it), stow better and give extra area in the roach. The cost of modifying an existing sail and especially one of any age would IMO be better put towards a new sail.
We used to own a W33 Ketch fairly similar to your Countess 33 and had both a FB main and mizzen, very effective. In that size the hardware is still fairly simple with no need for roller bearing mast cars for example, ours had simple teflon impregnated mast slugs with an extra external face to take the compression from the battens.
A new sail of any type will transform performance over an old out of shape one and a FB one will add even more oomph.
We have a fully battened main on our lovely Trapper 500, what is the extra hardware cos when we take it off to wash, we just put the old main in its place. Am I missing some hardware, or just the point?
I believe the other respondants are possibly referring to batten cars and maybe a external track for the mast. IMHO this is usually only required on bigger yachts.
In fact, we had a fully battened main on a 19 metre mast that worked fine with the teflon slippery slugs - so we needed no extra hardware either.
I don't see why it should be very expensive to convert to full battens. I'm sure when a sailmaker builds a sail they are one of the last bits to be added. You certainly don't need special hardware below about the 35foot mark. Agree with all the advantages mentioned by Robin.
FB main is harder to get up or down and to reef if not head to wind, and hard to depower completely in close quarters manoevering under sail. Expensive batten cars can help if you have a problem. I have had the experience of being unable to drop a FB mainsail with slides only in strong winds on a 33 footer.
BUT - it sets better in light airs and allows you to point a bit higher, and drops nicely into a set of lazyjacks.
FB is best for racing and fun sailing - especially if you have a reliable motor to keep the boat head to wind when sailhandling. Conventional is probably better for long distance cruising.
All IMHO of course. I also think is that it is rarely worth radically altering an old sail - better to keep it as a spare, bite the bullet and buy a new one. Before deciding, if possible go sailing on a similar boat with an FB main and see if you like it.