The son of By`shon Did not go from Tzur to Tzidon on erev Shabbos although the market day in Tzidon was on Friday. His children came to Rabbi Yochanan saying that while their father was wealthy and could afford to be absent on market day, they couldn’t afford to do so. They therefore wanted to change the minhag of their fathers. Rabbi Yochanan told them that a Minhag of your father must be kept even when circumstances become more difficult.
It is important to note that although circumstances had become more difficult, the purpose of the Minhag was still there. Since erev shabbos is a day better spent preparing for shabbos, abstaining from going to the market would be a perpetuation of the Minhag.
However, if the market day were to change to Monday, they would no longer be required to abstain from going to the market, as doing so would not be a perpetuation of the Minhag.
The Minhag was to abstain on Friday. It was not a blanket Minhag to refrain from going to the Market.