Question

what did the jacobians change their pattern of dress

Answer 1

Jacobins wanted their selves to send out from the rest of the fashionable people of the society (the nobles and the clergy). That's why they started wearing stripped trousers without knee breeches and came to be known as sans-culottes. In addition they wore a liberty cap too. However the ladies were not allowed to wear the Liberty caps.

Answer 2

The Jacobians decided to wear long striped trousers to set themselves apart from the fashionable sections of society, particularly nobles, who wore knee breeches. It was a way of proclaiming the end of the power wielded by the wearers of knee breeches.