Changing of the Guard is a 11:30a. If I stated until the end (12:00??) Would I have time to make the train to Paris at 2:04p?

You need to be at St Pancras by 1:15 (1:30 at the very latest) so time really isn't an issue.

But do you really want to stand outside in massive crowds for 1.5-2 hours w/ your luggage? You need to get to the Palace quite early to see much and then you just have to stand there doing nothing.

Not such a great idea IMO.

Didn't think about luggage..thanks.
Maybe I could get a hotel close to the Palace, leave luggage at front desk. See changing, get luggage and be at St Pancras by 1:15. Is that doable?

We were there two weeks ago and were disappointed that we could see NOTHING. Huge crowds. Unless you are seven feet tall you won't see a thing. Also, be advised that the ritual only takes place every other day in the fall. It was a big waste of precious time.

I prefer the Guard Changing at Horse Guards, Whitehall. And it starts earlier, too, giving you more time.

The Changing of the Guards is mildly amusing IMHO - but you MUST get there early enough to be in the first 2 or 3 rows of people or you will see nothing. and if you're short you may well still see nothing. This involves getting there at least 2 hours in advance of the activitiy.

I did it once - about 30 years and many many trips ago - and am happy I did. but certainly wouldnt do it again.

I'm on board with everyone else... it is a bit chaotic.

I would recommend some alternatives:
- Horse Guards
- changing of the Guards at Windsor Palace - are you going there by any chance ?

For what you see, it just isn't worth the aggravation.