DangWell, I tried to upgrade the front brakes last Tuesday, but failed miserably. I started with the left front wheel, loosened the 300mm caliper flexible line from the hard line, and there it was my first warning. the hard line didn't come off the flexible line fitting, I undid the nut all the way, the hard or flex line moved, but it didn't wanted to get out. after fighting it for a while, I finally pulled it apart. I tried to install the flex line with the 320mm caliper and it was impossible to thread-in the hard line nut into the flex line. The thread sizes are the same. The hard line nut seems to be cross threaded. After fighting with it for 3 hours and wasting 1 liter of brake fluid (filling the reservoir to avoid getting air in the lines) I tried to install the original caliper back. After trying 3 times (with terrible pain on my hands and fingers) the flex line threaded in and tightened completely. It didn't leak, I bled it. and reinstalled everything.
It was a very dissapointing experience......... Not sure if I should try again or leave it like that and be done with it.