Recently I moved to Eclipse Mars IDE from Indigo to get support for Java 8, While trying to use to older workspace with Mars I received the following warning when I opened the software for the first time,
Workspace was written with an older version of the product and will be updated. Updating the workspace can make it incompatible with older versions of the product
Are you sure you want to continue with this workspace?
As there may be some configurational changes in files related to workspace in the newer version of the IDE which may cause issues, I rather creating a new workspace and imported the project there.