Is there a way I can figure out what my trajectory would be if I zeroed with one load and switched to another without actually going out and shooting at different distances?  I've seen lots of places on the web to calculate trajectory, but I haven't found anything that lets you calculate this scenerio.

hmm...that is gonna be difficult to do because acceleration of the bullet is not constant so it would require a differential equation which is pretty tough shit, so if there is a way to do what you want, it's gonna be a like a rule of thumb or somethign and it won't be precise

There is a way you can 'guesstimate it'.

Run a ballistic calculator with your other ammo's figures based on the zero you shot with the first load.

No it won't be perfect but it will be fairly close.  At closer ranges (say less than 100 yards) the rounds are traveling fast enough that if you zero at 50 yards with 55gr M193 - you'll be pretty much on with the 77gr Mk262 [b]AT THAT DISTANCE[/b].  

True reality is you'll be of by a very small fraction of an inch - unless you're shooting a bench rest rifle you'll never notice the difference.

BTW the one thing the ballistic calculator's won't tell you is the difference in drift due to the spinning of the round.  If you zero (and confirm at long range) with the 55gr round you'll find the heavier rounds will drift off a bit to the right IIRC at extended ranges (250m +).