In the last stanza in Lecha Dodi, Ashkenazi Nusach Sfard, say on a regular Shabbat - "Gam BeRina", whereas on Yom Tov that falls in Shabbat we say "Gam BeSimcha". What is the reason for this change? Is there any Kabbalistic source for it, as well? BTW on Rosh Hashana that falls in Shabbat there is no change (i.e. we say "Gam BeRina"), and I would like to understand why?