I was often confronted to this issue where i had some changes to do that don't impact the track and the ghosts but some decorations, MediaTracker elements, etc. And when you do, you normally need to revalidate your map, which mean you may never do the time your did before for instance. So here's a solution i found using OpenPlanet.
- Open your map in the editor.
- Do any changes you wanted to.
- If not already done, press F3 (or the key you binded) to see the OpenPlanet menu.
- Open the System menu and click on Nod Explorer.
- Click on the arrow in front of "CTrackMania".
- Search for "CGameCtnChallenge" and click on the arrow in front of it.
- Search for the 4 lines with -1 (TMObjective_...). These are the times required to get the medals, you can change all of them or just the author time (TMObjective_AuthorTime). You need to put a time in milliseconds, e.g. 60000 for 1 minute. You can find the time of your best replay by opening the replay with a text editor (like Notepad++, etc.) and searching in the first few lines (header) something like that : "<times best="XXXXX" respawns="0" stuntscore="11" validable="1"/>".
- Close the Nod Explorer and click on the validation icon.
- You can press your ESC key to go back to the editor. Your map is validated (icon should be green). If you only changed the author time, you can then change the others times in the Advanced menu -> Change the map goals.