I will be taking the Eurostar from London to Paris. I will also have Global Eurail pass. My understanding is that if I make a reservation, I can change it once before departure. Is this correct? If so, how do I go about doing so? If I arrive in time for an earlier train (i.e., before my scheduled reservation), can I simply make the change at the London station? And can I activate the pass there as well? Any tips would be appreciated.
The pass allows you to purchase discounted Eurostar tickets, but you have to check the conditions of the exact ticket you purchase to see what the conditions for a change might be. Generally, you have to pay a higher price for a "flexible" ticket which allows certain changes and refunds. It appears that the Eurostar ticket purchased as a passholder allows one exchange within Europe, but no refunds.
Yes, you can validate the pass at any rail station.
Eurostar tickets are best purchased as far in advance as possible to get the best price. They quickly go up in price after the "discounted" tickets all sell out. As an example, I purchased tickets for late June at a price of $101 per person from London to Paris. Today, you can't touch the same train for less than around $250 per person.