Does anyone know how to change Music Wallet's directory? Currently it's downloading blockchain to my AppData in C:/ and it's almost full
Or can we change online wallet ID same as my desktop wallet? Because I don't want start to mine it again

I do not run MUSIC on Windows, but perhaps I can help.
To change the data directory on Linux, for example, this is what I do ...
~/go-musicoin/build/bin/gmc --rpc --rpcapi="db,eth,net,web3,personal" --rpcport "8546" --rpcaddr "127.0.0.1" --rpccorsdomain "localhost" --datadir "/home/user/musicoinHighDiff" --port 30304 console
... Take note of the
--datadir "/home/user/musicoinHighDiff" in the above command (towards the end). This would tell MUSIC to usehome/user/musicoinHighDiff/ as the data directory.
On Windows, I would presume you would
add something like ...
--datadir "D:\SpotWithMoreRoom\MC\"
... to the shortcut or command you are using to start MUSIC.
Last I looked at the online wallet, I did not see the ability to import keys. So, I think no. But I may have overlooked it.
Again, I do not use a Windows MUSIC wallet but, the info should be relevant. I hope this is helpful!