First of all I do not recommend keeping any large sum of money on Coinbase, it is more secure if you use a hardware wallet or good desktop wallet.
I do not think you can swap between the two on coinbase but there are several ways to achieve the end result.
You could sell a portion of the BTC on coinbase and then use the proceeds to buy LTC
I think coinbase subsidiary GDAX allows for trading directly between (This requires setting up more accounts, I dont recommend)
The last and probablly best option would be to use a service like shapeshift, changelly or blocktrades.
For example in blocktrades.us you dont need an account you just send the amount of BTC you want to blocktrades and input your receive LTC Address and it does the swap, with a fee.
Look into shapeshift and changelly and see which works best for you.
https://shapeshift.io/
https://changelly.com/
In the future we will not even need third party services
https://steemit.com/cryptocurrency/@digicrypt/atomic-swaps-future-of-crypto
I hope that helps
Thanks! I'm clicking the links as we speak/type! I'm still confused about wallets though - how can I have an offline wallet? How will sites access my money?
If your funds are stored online in an exchange or service like coinbase or bittrex etc...you are relying on that company to protect your crypto. This can end badly like the infamous Mt Gox hack where everyone lost everything stored on the exchange.
You can download a desktop wallet like jaxx or exodus, that puts you in control of the private keys and noone else, however your pc could still be compromised.
What I personally do is use a hardware wallet. I use Keepkey and there should be a link to it at the bottom of my post. You could also use a product like the ledger nano or trezor, I personally like and use keepkey.
This is a physical and very secure device that holds your crypto. To use it (in the case of keepkey) it is pretty simple you download a google chrome app, plug in your device, open the app and you can spend your crypto! Keepkey is nice because you can view all your accounts at once and even create several btc accounts, one for personal use or one for business.
The best thing you can do is research, and find what works best for you!
Well, according to most sites, KeepKey is more popular than the others. I tried to buy it, but it's on backorder. How do you get your bitcoins INTO the wallet?
Yes, I have noticed they are out of stock at the moment. You just send them to the BTC address produced by your keepkey wallet, like you would with any transaction.