Just as a little supply of information regarding low cost rostering. Here in orange land, I have not once had a day off changed and the roster for existing bases is still 5 earlies, 3 days off, 5 lates, 4 days off for 17 weeks followed by 4 weeks of random roster with minium 10 days off (have been given 12 the last 4 reserve periods). All new bases and FOs (fixed pattern available in most bases after 1 year as SFO) are random roster, but as far as I am aware, rostering can not touch your days off once they are rostered.
I have flown into a day off maybe a handful of times in just under a decade. Never by more than half an hour.
This is not meant as a pissing contest, but there seems to be some confusion as to how rostering is in the low cost world.