Triple to double
 

What is the extent of the new parts required to convert from Shimano triple to Shimano double on a road bike most of the existing parts are Ultegra or 105.

Want to go with the new 34-50 set?

if this is a new external bearing bike, and both cranks are shimano, you might
be able to keep the BB in place, since the tube spindle is part of the right crank assembly.

If there is a BB packaged with the new crank, good. set it aside for future service..
buying separately later will be priced higher.

On the cheap- remove inner chainring and adjust the low limit screw to keep the FD from derailing the chain onto the BB shell. Instant double.

+1. By far the fastest, easiest and cheapest method.

Not sure exactly what you're starting from but if, for example, you have a FULL shimano Ultegra 6700 triple kit per shimano's specifications and you want to go to full double, you would need:

FC-6700 or FC-6750 crank
FD-6700 front derailleur
ST-6700 brake/shift lever
CN-6701 chain (since shimano specs the 6600 chain for triple configurations - if, somehow you already have a 6701 chain, you're good to go)
RD-6700-SS (assuming you currently have the RD-6700-GS, the SS would work better in the double config, but the GS will still work)

If you're not fit and thin enough to run a 39x21, 39x23, or 39x26 small gear (assuming 10 cogs - 19, 21, and 23 are the numbers with 9 and 18, 19, 21 with 8) depending on whether you prefer an 11, 12, or 13 starting cog you'd benefit from keeping the triple and swapping the cassette through tighter spacing between cogs and less front shifting than with a compact double.

you don't need all of that. all you need is the crankset. usually a good idea to swap the bb while you're there though.

i have done this btw. your 3rd shift on the shifter will be like a trim and you need to do some derailleur adjusting but that's all.

I could not adjust my derailleur so that it made it to both rings on my 50-34 crankset without often dropping the chain until I gave up trying to use a triple front derailleur (Campagnolo Racing-T) and had it working in a few minutes with the correct double (Campagnolo Chorus CT).

I've never noticed any shift quality differences between short, medium, and long cage derailleurs on the same bike.

I could see the larger units wearing out their upper and lower pivots faster due to the added leverage, although the sixteen years I got out of my last medium unit was long enough.

My experience also. I've always considered the recommendation that short cage derailleurs shift faster, more precisely, etc. to be a myth. Yes they are slightly lighter and have more style points but I defy you to demonstrate a measurable performance difference.