I suspect that they are going to meet Remilya, aka the Master of the Scarlet Mansion, so Cirno wants to look somewhat respectable and impressive to meet such "important" individual.
JusticeItEasy said: I suspect that they are going to meet Remilya, aka the Master of the Scarlet Mansion, so Cirno wants to look somewhat respectable and impressive to meet such "important" individual.
Err, got to this late but Sakuya wanted cirno to look respectable before the charismatic Remilia and told her to change forms.