Lens-to-headlight seals are going to be a bit squashed from all the years of being under pressure. It is not vital to change them when you change lenses. You can probably reposition them a bit then you change the lens so they get squashed in different position this time. I changed mine as I wanted complete air tightness. The cost of changing the reflector damaged by moisture is much higher.
My outer surrund rubber gasket was in perfect condition, so I didn't change it. It won't get damaged when you remove the metal clamps. Metal clamps I ordered new as mine were a bit rusty. You can go to powered paint people and powder paint them. This is what I will do with the old ones which i removed.
Yep, your new LED bulbs are much bigger, so may not fit the hole.
Also, be careful with potential heat problems.
Let me know how you're getting on with them. Very interesting.
I purchased similar LED 'lens' bulbs for reversing lights a couple of years ago. They use CREE leds. They are large and work like a real torch. The fit into lenses without modifications. It created a great difference when reversing.
EDIT: Here's the link of what I actually installed, but even better models may be available this days with all the advancements in LED technology.
http://www.ebay.co.uk/itm/2-PCS-1156...-/271265839796