MYOG 4season Tent

I would like to replace the current fly with a cuben fly

You're going to want to track down Mr. Dan Durston for that task. He made a bonded cuben fly for his tent and it turned out very nice.

For me, being quite new to sewing, the fly would be easier to do than the liner, but if you know you way around a sewing machine than most likely either won't be too hard. As long as you have the current fly and liner, then you've already got the pattern.

As for whether or not it will be fine for winter, depends on conditions/area, but some guys here use tarps in the winter.

It might be more trouble than it's worth to modify existing tent.

Just make what you want – it'll end up being more what you want and weigh less.

I would like to replace the current fly with a cuben fly

If you are doing that for fun (as a hobby…) then go ahead it may work out.
On the other hand if it is about not having to buy another shelter you might be surprised at how much it will cost.
Yes contact the ones that have done that before.
Don't forget that it isn't just the cost of the Cuben , you need to saw/bond it ,get all the trimmings like zippers and maybe even a new sewing machine or at least new needles. It all adds up…
Franco