So far, it hasn't been too much trouble. I managed to fix single includes in the main branch, and all deprecated functions in libpurple core too. Finch was super-easy and required almost no work. What was left was the Pidgin UI.
The main Pidgin UI required a bit more work. Replacing GtkOptionMenu with GtkComboBox was pretty simple, just a couple preprocessor checks needed. Using GtkTooltip instead of GtkTooltips was also pretty painless, but there's still one matter left. The headline text that goes in the buddy list for mail notification used to do a hack with the tooltip to colour it correctly. At the moment, it ends up grey because I'm not sure how to get it working.
There are still a couple big changes left. I still really don't know how to write UI Manager stuff, and it's hard to find good examples. It only has to be done twice, for the buddy list and for the conversation window, but I'm not sure how much that really means. Fortunately, we are not heavily GNOME dependant, so that's probably the biggest change.