had a similar problem when upgrading from 19->21. I removed the old associations from the registry and then associated the .pde again.
// warning usage of regedit can cause serious damage to windows registry (on the other hand it is sometimes the only way)
Start regedit.exe
Check the following entries: - CTRL-F = find .pde - use F3 to repeat search
Computer\HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts.pde\OpenWithList
and
Computer\HKEY_USERS\ - some number - \Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts.pde\OpenWithList
as -some number- might differ you should use seach.
I removed the entries here so windows "lost" all associations. When opening an .pde file windows will ask for the association, use browse to select Arduino21 and it worked as intended.