alright I am going to give you as much help as a can over the internet. however you need to remember this will not be as good of help as it would be if you were to see a boot fitter. Ok, first take your current boots, pull the liners out and slide your foot in the shell, Move your foot so that your toes touch the front of the boot, with your foot centered in the boot width wise. Now lean forward, take your hand and use your fingers to measure the distance between your heel and the back of the boot, The idea fit is 2 fingers widths. you don't want to go over this but it is ok to go under, at 1 finger it will be too small. at 1.5 fingers it will be ok. So anyway, measure the distance, if you can get one finger in with quite a bit of excess space (up to two fingers), your current boots are fine, and new boots would be too big. If you can get from about half a finger (guess) upto one finger in there (slight extra space), go to a size 27/27.5, If there is really no room whatsoever go with the 28. Think of it this way when you go up a size the boot gets 1cm bigger, so the number of sizes you go up should be equal to or less then the number of cm you are short from 2 fingers.
That is the best that I can do with out being there, I am not guaranteeing that it will fit, but it would be better then guessing and going with the 28 when it is too big. But think about it your foot went one size up, so why would your boots go two sizes bigger?