Scenario:
Project A has existed for some time and is licensed using Apache 2.0.
Big Project B is going to use and perhaps redistribute Project A and requires a contributor's licensing agreement (CLA) (not copyright transfer). Project B will offer it's works under the GPL and as proprietary software.
Where does that leave project A, of which there will be two versions? Will the applicable license simply be a function of which project a user downloads the code from?
It's not clear who the copyright owner(s) in project A are, nor who is signing the CLA, and whether the copyright holder(s) is/are the person(s) signing the CLA.
The simplest option is that project B distributes project A's code under the Apache 2 license, whilst continuing to distribute the rest of it's code as GPL. This is perfectly possible, as the licenses are compatible, assuming it is GPLv3. (See http://www.gnu.org/licenses/license-list.html )
If project B makes modifications, they would likely be licensed only under the GPL (depending on what the CLA) says. This would mean those modifications would be unusable in project A, unless the copyright owner also chose to contribute them to project A under the apache 2.0 license.
A bit more information would allow a clearer answer to be made.