I'm working with Flex SDK 3.2 since 2010 with Flex builder 3 with eclipse ganymed. Now I want tu use Flex SDK 4.6 Cant I Open the workspace using a new Flash Builder plugin into new eclise mars.1 ?
You can do this, sort of. I support a long running application that has it's front end in Flex and it's backend on JBoss/Oracle and we have to use this gimmick to keep the tools happy.
Since you have 3.7 installed with FlashBuilder 4.7 already installed, simply install (unzip) Mars.1 somewhere else then copy the file fb-4_7-plugin-encoded.link like this:
cp C:\eclipse-juno-64\dropins\fb-4_7-plugin-encoded.link C:\eclipse-mars-64\dropins