Upgrading is not supported at this time, due to the fact that there is no simple data conversion process. WEM can (recommended) use a different database however, and if configured as such it would be possible to retain the old SWEM data in the database and have new WEM data in the WEM database. This would also support taking WEM down and standing WEM back up if necessary.
This particular configuration is not currently supported, however it is in theory possible that both could live on the same server. They use a lot of the same technologies and most of those which are shared are backwards compatible. The only obvious issue is that if running with SPIRIT support enabled on WEM, both would need to expose a web service endpoint, and the default configuration for each is to use port 80, which one of them could not do if both are running on the same NIC. A dual homed network configuration could possibly overcome this, but again it has not been tested. However, it is important to note that this default port configuration can be changed.