Solid: from XMPP to PSYC
The protocol switch is achieved using the strategy described at http://switch.psyced.org
psyced implements this reliably.
See also PSYC as Jabber S2S.
Liquid: from PSYC to XMPP
PSYC has a negotiation feature where you can state which other protocols your node supports. Since we haven't had the need, there is however no precise procedure on how to fulfil such a switch. Currently it only serves the purpose of advertising other options.