As a rough guide because I'm not HTC it goes something like this. You need to win it to get a new terrain OR it's usually rotated every new TOD (1st of the month). They did used to rotate at a set time but some folk complained about that.
Usually terrain does not change on new tour start, most if the time (if not always) there is only an arena reset.
We had a weekly wednesday terrain change before the arena's were split. I don't remember having ever seen any complaints about this.
If you want to have a new map, you will now have to reset the current one. Does still happen frequently with small maps, but is very hard to do with the two large ones we have, especially when it's the LW Orange arena. It's next to impossible to sustain the offensive long enough to get the required number of fields from both countries.
I'm all for the return of the old, scheduled weekly wednesday reset. When TT arena reverts back to normal operations the map could be changed.