In 2.1.4 we upgraded the svnkit library to 1.2. This library will upgrade all svn working copies to 1.5.

There is a flag available svnkit.upgradeWC=true which will prevent it from upgrading any existing copies to 1.5 but all new checkouts will be 1.5

This should not cause any compatibility issues with the subversion server and should only surface as a problem if you are using a 1.4 client to perform operations on the code bamboo has checked out (e.g as part of your build)