You should be able to use a different version of RE:Flex without loosing anything in your project.
The plugins are installed here:
CC and CC 2014
Windows: C:/Program Files/Adobe/Common/Plug-ins/7.0/MediaCore
Mac: /Library/Application Support/Adobe/Common/Plug-ins/7.0/MediaCore
You can always to be sure,
1) Make a copy of your AEP
2) move the RE:Flex folder somewhere outside of the plug-ins folder, install the current version and check.
The only issue is if someone saves from RE:Flex 5 to 4. For example RE:Flex 5 has a GPU menu, that value will not be saved. Similarly saving from CC 2014 as CC 12.0 and loading in CC AE 12.0 should work.
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