You really need this answer, right? ;)
Setting the status to Retired and back to Active will not change the "is managed" flag. Looking at the DB, the "is managed" item seems to be an association, anyway, it can be modified directly via SQL.
If you change the status to Retired, your inventory information MIGHT get purged, but that depends on your CMDB configuration. Look at the Task in > Manage > Jobs and Tasks > System Jobs and Tasks > Service and Asset Management > CMDB > Inventory Cleanup
Usually I set this up to purge the inventory nightly. In general it's suggested to turn this on, as it will save you licenses of retired computers.
This being said, I'd like to understand why you only want to change the "is managed" flag but keep inventory data and the machine active. This has different effects:
1. Active machines are members of all filters, thus you have additional processing time to update them
2. You keep on consuming licenses for the different solutions
For customers who have asset and CMDB, we usually setup an "internal" connector which takes the data which is needed to be kept from inventory data classes and write those to Asset/CMDB custom data classes and a weekly schedule.
This way, we have all the data we need for future use in the asset information and can change the status to "retired". That will clean out inventory data and release the license, but our "most important" data keeps on residing in the CMDB, as it's in custom data classes.