Old Trim over new windows and siding? (Is this even possible?)

[Curt Bixel]

I am in mid rehab on a home built in 1910.  The windows and current trim are in bad enough shape that I am pulling both the windows and the exterior trim and installing new windows with a nailing fin and integrated J-channel.  After the new siding is installed, I am hoping to trim out the windows on the front.  

Since I will be removing the original trim from all over the house, I will certainly have enough trim that is still in good condition to find enough to trim out the few windows on the front with the original style trim.  I could also do a fairly good job in matching the trim with some new PVC trim or Polyurethane trim.  (I did not really like any of the decorative trim options that could have been included in the manufacturing process of the window.)

My question is how would the old trim, or the new PVC trim be attached?  I am that after the vinyl siding is property installed into the J-channel, that the trim could simply be attached by placing screws through the face of the trim, through the vinyl siding, and into the wood underneath.  This would be easy, but I am concerned that this would interfere with the movement of the siding that is supposed to be allowed as the siding expands and contracts with changed in temperature.  

Also, I wonder if there is any way to plug the triangular gaps that occur between the siding and the back of the trim boards.  I am sure you could fill this with caulk, but this might also prevent the desired movement of the siding that is supposed to allow for thermal expansion and contraction. 

[Peter M.]

@Curt Bixel put the trim on first then run the siding up to it. It gets rid of the triangular pieces and no nailing/screwing through the siding.