I'm using geth on a linux VM, and Ethereum Wallet on my Windows. I had no problem at all switching to Ropsten on linux, but my Wallet refuses to change.
I used geth --datadir /path/to/testnet/data init genesis.json; geth --datadir /path/to/testnet/data --networkid 3 console, it managed to sync but when I run geth --testnet console it's back on Morden, same with the wallet.
As I wanted to change the Wallet, I used the version of geth in Users\me\AppData\Roaming\Ethereum Wallet\binaries\Geth\unpacked, but I don't know if it's the right one. I'm also not sure what /path/to/testnet/data is, I used ...\Roaming\Ethereum\testnet\
I used this blog post, this question worked on linux but not windows, and this question is an unrelated problem of absolute and relative paths.
Update : Honestly I'm speechless. The wallet seemed to work (synced to ~60k blocks) even though I don't think I did anything useful, and now it's trying to download the 1,800k blocks of the old blockchain back. Why.
Update2 : I managed to get work done by restarting my wallet everytime it went crazy and back to the 1.800k blocks, and I realized my geth console (not even on the same machine) went crazy at the same time, also setting top block to 1.800k. I don't know if it's syncing on Morden or if it's just a bug and it still uses Ropsten, I can't tell. It seems to stay at 69k.
Update3 : It doesn't work anymore to close and restart the wallet. Now it looks for peers and downloads the 1,800k Morden blockchain. If I start geth alone, (with no parameters other it does show networkid=3 but can't find the blockchain data.
Update4 : I found a temporary fix. I can alway rerun the whole switching (get --datadir ... --networkid 3 console), but I lose all my wallet data (contracts and have to reload the blockchain. The "fix" I found is after reseting and starting Eth Wallet, I actually run geth --testnet console with a command prompt in the background, and it seems to keep the blockchain correctly synced, avoiding the Wallet to fall back to syncing Morden (which is weird because I couldn't run geth and the Wallet at the same time before). And when it does, just run exitand geth --testnet console again. I don't know what happens after you reboot your pc but at least I can work 2+ hours in a row without losing my data. I'm starting to think it's not going to go away until the next version of Mist...
########## BAD BLOCK #########instead of ########## BAD BLOCK #########. You can do that using grave accents : ` – Teleporting Goat Dec 02 '16 at 14:21