I didn't have to change any resistors on the relay board. The board is supposedly 5V. If the pi outputs 3.3V, that is usually enough to be considered HIGH on 5V systems. If it doesn't work, you can try wiring in the resistors, or even try powering the relay from 3.3V, which might make the triggering voltage a bit lower.
It's definitely possible that they've changed the board (mine is several years old now) and they have a hefty pull-down installed (which would make the 3.3V effectively smaller, which might not flip). There's no harm in trying it without the resistor though.