Contact Marriott ressies & get them to sort it out before you head over. Mention that you've got the screenshots as verification. They should honor it & sort it out in your record. Better to do it before you go over. Also, make sure your bring the screenshots w/ you/back-up documentation when you arrive.

Given your amended reservation is showing both your original and amended upgrade fee in the same ressie, it might just be a glitch in the system.

Granted I haven't seen the junior suites at St. Pancras, but I think 205 upgrade fee is a bit much if they're changing the upgrade fee! I could see the 120 since some properties (Grosvenor Square for example) charge 75 for exec room.

Cheers.