While Alfresco 4.2 can certainly use a content store written by 6.1 (really, nothing in Alfresco's content store structure has changed since at least 3.2), the DB is another matter. While there haven't been significant changes to the schema relating to core tables and columns, I don't think you will find anyone that would be able to confirm it as possible / risk-free.
Generally, I also do not understand why that would be necessary at all. If it were to access some information / data that is now inaccessible somehow after the upgrade, then I would simply recommend to use the pre-upgrade backup of your 4.2 DB and set up a separate install to do that, without messing with your already upgrade DB. Even if it were to work, by accessing an updated DB with an older Alfresco version, you are risking the introduction of invalid data constellations because of bugs in the older version, which the upgrade had already fixed. The upgrade patches will not be run a second time, so any such errors you reintroduce will stay there and may destabilise your 6.1 system.
My doubt is that a few time after the migration, I need come back to old environment because other application had some problems. In my other applications, I can dump the DB and it work in old environment/applications (no change schema).
I don't know if there is a strategy to back to older version in Alfresco.
How can I read Alfresco 6.1 ContentStore in Alfresco 4.2? I know ContentStore and DB are strongly related.