Switching hosts in mid-game is not recommended. If you are familiar enough with the hosting process, familiar with HOST, and familiar with PHost, then you know the answer to this question. If not, don't switch.
Okay, if you insist on it: technically, you can switch from HOST to PHost and back at any time, provided you use a ship list understood by both; the file formats are the same. However, you have to manually convert the configuration. You'll likely piss some players, though.
One caveat to watch out for is the auxdata.hst file. If you switch from PHost to HOST, the file will remain there although HOST does not maintain it. This may confuse some programs. In addition, when you switch back to PHost, it will find the stale file lying around. Therefore, delete auxdata.hst when you switch from PHost to HOST.
