It will happen automatically when you pay someone who only accepts Bitcoins from the other gateway. There are several different ways you could form transactions to do this, but I think the client doesn't currently give you an easy way to get to any of them.
In the Ripple system internals, people can create offers whose sides are the same currency but different issuers. However, you can also achieve the same effect without having to go through an offer (and likely at a lower cost) by using a circular payment path with no exchanges.
Again, you can't do this with the client currently, but here's how it works on the server side:
You have 1 WeExchange BTC. You want 1 Bitstamp BTC.
You form a payment path like this:
You hand this payment path to the server, telling it you want to get 1 Bitcoin out.
The server finds the cheapest way to do it. Most likely, it will find one or more people who have extended trust to WeEx/BTC and hold Bitstamp/BTC. You can use any of those people. (For safety, you can pick three to make sure your transaction goes through.)
With the three payment paths the server found you, you form a "Payment" transaction. Assuming all three of those people don't change their trust or balance settings before your transaction applies, the exchange is complete. Someone in the middle has your WeEx/BTC and you have their Bitstamp/BTC.
Note that if there's an exchange offer that gives you better than 1-to-1, the pathfinding scheme will use that instead. So you could wind up with more bitcoins out than in. (Though it's not likely.)
This actually will not allow you to get 1 Bitcoin out because you would have to pay two transfer fees to transfer the Bitstamp and WeEx IOUs. The total of the transfer fees is something like .7%, so you'd only get .993 Bitcoins out.
Instead, if you're not in a hurry, you could drop your credit limit to WeExchange to zero. This signals that you want to get rid of your WeExchange IOUs. Anyone picking a payment path like above in the other direction will try to accommodate you. Then they'd pay the transfer fees. (The current pathfinding algorithm doesn't always do this, but the new one scheduled to replace it shortly will be better at this.)