If I remember correctly there were various changes that we had noted were likely to require more changes in Geany, IIRC things like the change of rectangular selections to multiple selections.
I haven't paid close attention or looked lately, but I believe the LongTerm3 branch of Scintilla that we're currently using has most/all of the applicable changes backported from Scintilla head.
Also the change of the build system to make lexilla separate.
Between the lexer library and the compiler change, I suspect the build system will account for most of the needed changes, yeah.