Wil release M7 affect my previous activities on release-M6 ?
You may have some rebasing to do but I don't imagine it will be that much work. If you get stuck then give us a shout! This is pretty normal for alpha projects. Expect breaking changes until we reach Corda 1.0.
To add to what Roger said; you are entirely free to continue using M6, I'd recommend upgrading though since we won't be backporting any fixes to M6.