To move ATOM stakes from one validator to another, it's called Re-delegation.
1. On your browser, go to https://lunie.io
2. Click on Wallet and click Sign In
3. Select Use an existing address
4. Choose Use Ledger Nano
5. Connect your Ledger Nano, enter PIN code and click Sign In
6. Select Staking menu, click on Validators
7. Select a Validator which you wish to re-delegate to
8. Click Delegate
9. Click on the From dropdown list to select the validator to re-delegate "from" (assuming you know which one it is that you've previously delegated to)
10. Enter in the amount of ATOMs to be re-delegated
11. Review Network Fee and total amount of transaction, and click Next
12. Open up the Cosmos app on your Ledger Nano and click Send
13. Review and sign the transaction on your Ledger Nano
Congratulations! You have now successfully staked your ATOMs with a validator.
Re-delegation refers to the action of transferring ATOMs bonded with one validator to another validator. This is generally associated with re-balancing or diversifying your staked tokens across a number of your chosen validators, or simply changing validators.
Re-delegation, unlike un-bonding, in general is not subject to a 21-day holding period. However there are some exceptions to the rules.
Serial Re-delegation, or is sometimes referred to as validator hopping, is discouraged. This means when one re-delegates from Validator X to Validator Y and immediately re-delegate from Validator Y to Validator Z, the action is blocked temporarily. The immediate Re-delegation from Validator Y to Validator Z, or to any other validators, is subject to a 21-day holding period. After this "cooling off" period, one can re-delegate to another validator and the same goes on where one cannot Re-delegate again until after 21 days later.
There's also a 7-time Re-delegation limitation from the same wallet between one validator to another. For instance, within the same wallet, Cosmos can process multiple Re-delegation requests from Validator A to Validator B, for partial Re-delegations (not the full amount), consecutively for up to 7 times. The 8th Re-delegation request is then subject to the 21-day holding period.