First off. Backup!
Download this
Zip file. It just contains some instructional movies and sym-linked folders to make things easier, plus a collection of fonts. Extracting the Zip doesn't replace anything on your system, you will do this manually.
You need to replace the font in two locations for this to work correctly. Once unzipped, you'll see;
'Resources' folder - all the bits you need.
'ProtectedFonts' sym-linked folder - this links directly to the actual 'ProtectedFonts' folder on your system.
'System Fonts' sym-linked folder - again this links to the 'System Fonts' folder on your system.
Open up 'Resources > Fonts > Myriad Pro' - Inside you'll see 'LucidaGrande.ttc' - this is actually the 'Myriad Pro' font renamed.
In another window open 'ProtectedFonts' and in another window open 'System Fonts'.
'ProtectedFonts' needs to be replaced first. So copy 'LucidaGrande.ttc' from the location above. In 'ProtectedFonts' delete the 'LucidaGrande'ttc' file (don't just paste and overwrite the file), you may receive a warning and may need to enter your Admin password. Once deleted, paste the font you copied above.
Next, do 'SystemFonts' - same procedure. Delete first, OK any warnings, then paste.
Once you're happy that everything has copied OK and all warnings and requests for Admin passwords have been acknowledged. You need to do a reboot.
After the reboot, the system font should now be in 'Myriad Pro'.
Next, and this bit is important. Open 'Disk Utility' and do a 'Permissions Repair' of your drive to ensure the correct permissions are applied to these fonts you've copied to the system folders. Another reboot and all should be OK.