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 legacy
PSYC has a negotiation feature where you can state which other protocols your node supports, such as XMPP, HTTP and SMTP. Since we haven't had the need, there is however no precise procedure on how to fulfill such a switch. Currently it only serves the purpose of advertising other options.