Open software licenses are generally non-revokable (unless you violate them). They can only change the license of future versions.

I'm afraid that they can always do that with or without a CLA. Apache is quite a permissive license.

I'm not an expert in licenses, but my understanding is that if someone makes a contribution under one license, then any program including that code needs to obey the terms of that license. The CLA allows them to choose any license (well, depending on the specifics of the CLA) they would like to distribute that code under in the future (although they'd likely need to continue to make available past source to meet the terms of previously released versions.

So if I make a contribution under some very restrictive license, but sign a CLA allowing relicensing, all bets for the future are off.

Apache license doesn't forbid anyone to integrate your code and release the combination under proprietary licenses. The only requirements are to keep the license file and the copyright notice intact, and to state changes if you choose to publish the modified source code.

Therefore, it doesn't make a difference to have a CLA or not. What you trying to avoid is already allowed as per the license.

However, some CLAs require you to declare the originality and ownership of your contributions. Maybe that's why they require a CLA to be signed for contributions.