You can upload replays to any forum post you make by simply clicking the "Attach image/file" button
Replays typically "break" after every patch. In other words, replays can typically only be used in-game between patches.
One way to try to use "broken" replays is by using the command line -allowIncompatibleReplays. to do this, in your Steam Library, right click on Company of Heroes 2, click on properties, Set Launch Options, enter -allowIncompatibleReplays and then hit OK! Although there is no guarantee this will work (or work properly if it does load your replay), it can sometimes extend your replay's life.
The best thing to do to ensure Replays last forever is to record them to video using some kind of third party recording software such as Fraps, XSplit, etc. and then upload them to YouTube!